diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..e3a996d
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,15 @@
+# sbt build artifacts
+target/
+project/target/
+project/project/
+
+# IDE
+.idea/
+.bsp/
+.metals/
+.vscode/
+*.iml
+
+# OS
+.DS_Store
+Thumbs.db
diff --git a/README.md b/README.md
index 3a7ea54..fbf256b 100644
--- a/README.md
+++ b/README.md
@@ -1,3 +1,260 @@
-# summercms-phrasebook
+# summer-phrasebook
 
-## SummerCMS i18n support
+Internationalization module for [SummerCMS](https://git.golem15.com/golem15/summercms). Provides translation loading, CLDR-aware pluralization, and parameter interpolation — the Scala 3 equivalent of Laravel's `Illuminate\Translation` + WinterCMS `Storm\Translation`.
+
+## Quick start
+
+```scala
+import summer.phrasebook.*
+import java.nio.file.Path
+
+val translator = PhrasebookTranslator(
+  initialLocale   = LocaleTag("en"),
+  initialFallback = Some(LocaleTag("en")),
+)
+
+// Register a plugin's translation directory
+translator.addNamespace("golem15.blog", Path.of("plugins/golem15/blog/lang"))
+
+// Simple lookup
+translator.get("golem15.blog::lang.labels.pluginName")
+// => "Blog"
+
+// With parameter replacement
+translator.get("golem15.blog::lang.welcome", Map("name" -> "Alice"))
+// => "Welcome, Alice!"
+
+// Pluralization (auto-selects correct CLDR form)
+translator.choice("golem15.blog::lang.posts_count", 5)
+// => "5 posts"
+
+// Switch locale
+translator.setLocale(LocaleTag("pl"))
+translator.choice("golem15.blog::lang.posts_count", 5)
+// => "5 wpisów"
+```
+
+## Translation file format
+
+Translation files use [HOCON](https://github.com/lightbend/config/blob/main/HOCON.md) (`.conf`), organized per-plugin, per-locale, per-group:
+
+```
+plugins/golem15/blog/lang/
+  en/
+    lang.conf
+    validation.conf
+  pl/
+    lang.conf
+```
+
+Example `lang/en/lang.conf`:
+
+```hocon
+labels {
+  pluginName = "Blog"
+  posts = "Posts"
+}
+
+post {
+  create_title = "Create Post"
+  edit_title = "Edit Post"
+}
+
+# Pluralization: pipe-separated forms (English: singular|plural)
+posts_count = ":count post|:count posts"
+
+# Explicit range syntax
+items_range = "{0} No items|{1} One item|[2,*] Many items"
+
+# Parameter interpolation
+welcome = "Welcome, :name!"
+```
+
+Nested HOCON keys are flattened to dot-separated paths: `labels.pluginName`, `post.create_title`, etc.
+
+## Key format
+
+Keys follow the WinterCMS/Laravel convention:
+
+```
+namespace::group.item
+```
+
+| Part | Description | Example |
+|------|-------------|---------|
+| `namespace` | Plugin identifier (optional) | `golem15.blog` |
+| `group` | Filename without `.conf` | `lang` |
+| `item` | Dot-path into the flattened HOCON | `labels.pluginName` |
+
+Examples:
+
+```
+golem15.blog::lang.labels.pluginName   -> plugins/golem15/blog/lang/{locale}/lang.conf -> labels.pluginName
+golem15.blog::validation.required      -> plugins/golem15/blog/lang/{locale}/validation.conf -> required
+system::validation.required            -> modules/system/lang/{locale}/validation.conf -> required
+```
+
+Keys without a namespace that start with `validation.` auto-resolve to `system::validation.*` for WinterCMS compatibility.
+
+## Parameter interpolation
+
+Use `:name` placeholders with automatic case variants:
+
+| Placeholder | Replacement | Result for `"alice"` |
+|-------------|-------------|---------------------|
+| `:name` | as-is | `alice` |
+| `:Name` | ucfirst | `Alice` |
+| `:NAME` | uppercase | `ALICE` |
+
+```scala
+translator.get("golem15.blog::lang.greeting", Map("name" -> "alice", "count" -> "3"))
+// Given: "Hello :Name, you have :count new messages"
+// => "Hello Alice, you have 3 new messages"
+```
+
+## Pluralization
+
+Two mechanisms, evaluated in order:
+
+### 1. Explicit conditions
+
+Exact values with `{N}` and ranges with `[from,to]` (`*` = unbounded):
+
+```hocon
+items = "{0} No items|{1} One item|[2,5] A few items|[6,*] Many items"
+```
+
+### 2. CLDR plural rules
+
+When no explicit condition matches, the engine selects a form by CLDR index based on the locale. Forms are pipe-separated, ordered by CLDR category:
+
+| Locale group | Forms | Categories |
+|-------------|-------|------------|
+| Chinese, Japanese, Korean, Turkish, ... | 1 | other |
+| English, German, Spanish, Italian, ... | 2 | one \| other |
+| French, Hindi, ... | 2 | one (0 and 1) \| other |
+| Polish, Russian, Ukrainian, Czech, ... | 3 | one \| few \| many |
+| Slovenian, Maltese, Welsh | 4 | varies |
+| Arabic | 6 | zero \| one \| two \| few \| many \| other |
+
+Covers 40+ locale groups. Ported from Laravel's `MessageSelector` (Zend Framework, BSD license).
+
+```hocon
+# English: singular|plural
+posts_count = ":count post|:count posts"
+
+# Polish: one|few|many
+posts_count = ":count wpis|:count wpisy|:count wpisów"
+
+# Arabic: zero|one|two|few|many|other
+posts_count = "لا مقالات|مقالة واحدة|مقالتان|:count مقالات|:count مقالة|:count مقالة"
+```
+
+```scala
+translator.choice("golem15.blog::lang.posts_count", 1, locale = Some(LocaleTag("pl")))
+// => "1 wpis"
+
+translator.choice("golem15.blog::lang.posts_count", 22, locale = Some(LocaleTag("pl")))
+// => "22 wpisy"
+```
+
+## Locale fallback
+
+When a key is not found in the requested locale, the engine walks a fallback chain:
+
+```
+requested (de-at) -> parent (de) -> configured fallback (en) -> core locale (en)
+```
+
+First match wins. If no translation is found anywhere, the raw key string is returned.
+
+## Namespace aliases
+
+Shorthand aliases for long plugin namespaces:
+
+```scala
+translator.addAlias("blog", "golem15.blog")
+
+translator.get("blog::lang.labels.pluginName")
+// => "Blog"
+```
+
+## Runtime overrides
+
+Override any translation at runtime (locale-specific):
+
+```scala
+translator.set("golem15.blog::lang.labels.pluginName", "My Custom Blog")
+
+translator.get("golem15.blog::lang.labels.pluginName")
+// => "My Custom Blog"
+```
+
+## API reference
+
+```scala
+trait Translator:
+  def get(key: String, replace: Map[String, String] = Map.empty,
+          locale: Option[LocaleTag] = None): String
+
+  def choice(key: String, number: Int, replace: Map[String, String] = Map.empty,
+             locale: Option[LocaleTag] = None): String
+
+  def has(key: String, locale: Option[LocaleTag] = None): Boolean
+
+  def allForLocale(locale: LocaleTag,
+                   namespace: Option[String] = None): Map[String, String]
+
+  def setLocale(locale: LocaleTag): Unit
+  def getLocale: LocaleTag
+  def getFallback: Option[LocaleTag]
+  def setFallback(locale: LocaleTag): Unit
+
+  def addNamespace(namespace: String, path: java.nio.file.Path): Unit
+  def addAlias(alias: String, namespace: String): Unit
+  def set(key: String, value: String, locale: Option[LocaleTag] = None): Unit
+  def clearCache(): Unit
+```
+
+## Module structure
+
+```
+src/main/scala/summer/phrasebook/
+  LocaleTag.scala              Opaque type for locale identifiers
+  TranslationKey.scala         ParsedKey case class (namespace/group/item)
+  KeyParser.scala              namespace::group.item parser with cache
+  PluralRules.scala            CLDR plural index rules (40+ locales)
+  Interpolator.scala           :name / :Name / :NAME replacement
+  HoconLoader.scala            HOCON -> flat Map[String, String]
+  FallbackChain.scala          Locale fallback chain builder
+  Translator.scala             Public API trait
+  PhrasebookTranslator.scala   Full implementation
+```
+
+## Dependencies
+
+| Dependency | Version | Purpose |
+|-----------|---------|---------|
+| `com.typesafe:config` | 1.4.3 | HOCON parsing |
+| `org.scalameta:munit` | 1.0.3 | Testing (test scope only) |
+
+No effect systems. No Cats, no ZIO. Pure direct-style Scala 3 on JDK 21.
+
+## Building
+
+```bash
+sbt compile    # compile
+sbt test       # run all 95 tests
+```
+
+Requires JDK 21+ and sbt 1.10+.
+
+## Roadmap
+
+- **Phase 2** — Integration: Pebble template extension (`|_` filter, `trans()`/`transChoice()` functions), Tapir endpoint for JSON translation export to Vue admin SPA
+- **Phase 3** — Tooling: `summer phrasebook:scan` (extract translatable strings), `summer phrasebook:export/import` (flat files for translators), dev-mode hot-reload
+- **Phase 4** — Database-backed translation overrides, compile-time key validation macros
+
+## License
+
+Part of SummerCMS. Plural rules derived from the Zend Framework (BSD license) via Laravel.
diff --git a/build.sbt b/build.sbt
new file mode 100644
index 0000000..6854715
--- /dev/null
+++ b/build.sbt
@@ -0,0 +1,20 @@
+lazy val root = (project in file("."))
+  .settings(
+    name         := "summer-phrasebook",
+    organization := "com.golem15.summer",
+    version      := "0.1.0-SNAPSHOT",
+    scalaVersion := "3.3.4",
+
+    libraryDependencies ++= Seq(
+      "com.typesafe" % "config"  % "1.4.3",
+      "org.scalameta" %% "munit" % "1.0.3" % Test,
+    ),
+
+    scalacOptions ++= Seq(
+      "-Wunused:all",
+      "-deprecation",
+      "-feature",
+    ),
+
+    testFrameworks += new TestFramework("munit.Framework"),
+  )
diff --git a/project/build.properties b/project/build.properties
new file mode 100644
index 0000000..73df629
--- /dev/null
+++ b/project/build.properties
@@ -0,0 +1 @@
+sbt.version=1.10.7
diff --git a/src/main/scala/summer/phrasebook/FallbackChain.scala b/src/main/scala/summer/phrasebook/FallbackChain.scala
new file mode 100644
index 0000000..7b621e3
--- /dev/null
+++ b/src/main/scala/summer/phrasebook/FallbackChain.scala
@@ -0,0 +1,50 @@
+package summer.phrasebook
+
+/**
+ * Builds the locale fallback chain for translation resolution.
+ *
+ * Order: requested locale → parent locale → configured fallback → core locale ("en").
+ *
+ * Example: `de-at` with fallback `de` and core `en`:
+ *   `Vector("de-at", "de", "en")`
+ *
+ * Duplicates are removed while preserving order.
+ */
+object FallbackChain:
+
+  val CoreLocale: LocaleTag = LocaleTag("en")
+
+  /**
+   * Build the fallback chain for a given requested locale.
+   *
+   * @param requested  the locale the user asked for
+   * @param fallback   optional configured fallback locale (e.g. site default)
+   * @param core       the ultimate fallback, defaults to "en"
+   */
+  def build(
+      requested: LocaleTag,
+      fallback: Option[LocaleTag] = None,
+      core: LocaleTag = CoreLocale,
+  ): Vector[LocaleTag] =
+    val chain = Vector.newBuilder[LocaleTag]
+    val seen  = collection.mutable.Set.empty[String]
+
+    def add(tag: LocaleTag): Unit =
+      if seen.add(tag.value) then chain += tag
+
+    // 1. Requested locale
+    add(requested)
+
+    // 2. Walk up parent chain (e.g. de-at -> de)
+    var current = requested
+    while current.parent.isDefined do
+      current = current.parent.get
+      add(current)
+
+    // 3. Configured fallback
+    fallback.foreach(add)
+
+    // 4. Core locale
+    add(core)
+
+    chain.result()
diff --git a/src/main/scala/summer/phrasebook/HoconLoader.scala b/src/main/scala/summer/phrasebook/HoconLoader.scala
new file mode 100644
index 0000000..aadce81
--- /dev/null
+++ b/src/main/scala/summer/phrasebook/HoconLoader.scala
@@ -0,0 +1,56 @@
+package summer.phrasebook
+
+import com.typesafe.config.{Config, ConfigFactory, ConfigValueType}
+import java.nio.file.{Files, Path}
+import scala.jdk.CollectionConverters.*
+
+/**
+ * Loads HOCON `.conf` files and flattens them into `Map[String, String]`.
+ *
+ * Nested HOCON keys become dot-separated paths:
+ * ```hocon
+ * labels {
+ *   pluginName = "Blog"
+ *   post {
+ *     title = "Post Title"
+ *   }
+ * }
+ * ```
+ * becomes:
+ * ```
+ * "labels.pluginName" -> "Blog"
+ * "labels.post.title" -> "Post Title"
+ * ```
+ */
+object HoconLoader:
+
+  /** Load and flatten a single HOCON file. Returns empty map if file doesn't exist. */
+  def load(path: Path): Map[String, String] =
+    if !Files.exists(path) then Map.empty
+    else
+      val config = ConfigFactory.parseFile(path.toFile).resolve()
+      flatten(config)
+
+  /** Load from a classpath resource name. */
+  def loadResource(resourceName: String): Map[String, String] =
+    val config = ConfigFactory.load(resourceName).resolve()
+    flatten(config)
+
+  /** Flatten a Typesafe Config into a string map. */
+  def flatten(config: Config): Map[String, String] =
+    val builder = Map.newBuilder[String, String]
+    for entry <- config.entrySet().asScala do
+      val key   = entry.getKey.nn
+      val value = entry.getValue.nn
+      value.valueType() match
+        case ConfigValueType.STRING | ConfigValueType.NUMBER | ConfigValueType.BOOLEAN =>
+          builder += key -> config.getString(key).nn
+        case ConfigValueType.NULL =>
+          // skip nulls
+        case ConfigValueType.LIST =>
+          // Store list values as comma-separated for simple cases
+          builder += key -> config.getStringList(key).nn.asScala.mkString(",")
+        case ConfigValueType.OBJECT =>
+          // Objects are already flattened by entrySet() traversal
+          ()
+    builder.result()
diff --git a/src/main/scala/summer/phrasebook/Interpolator.scala b/src/main/scala/summer/phrasebook/Interpolator.scala
new file mode 100644
index 0000000..a2d2b50
--- /dev/null
+++ b/src/main/scala/summer/phrasebook/Interpolator.scala
@@ -0,0 +1,41 @@
+package summer.phrasebook
+
+/**
+ * Performs parameter interpolation on translation strings.
+ *
+ * Ported from Laravel's `Translator::makeReplacements()`.
+ *
+ * For each replacement pair (key -> value), three substitutions are applied:
+ *   - `:key`  -> value (as-is)
+ *   - `:Key`  -> ucfirst(value)
+ *   - `:KEY`  -> uppercase(value)
+ *
+ * Longer keys are replaced first to avoid partial-match issues
+ * (mirroring PHP's `strtr()` behavior).
+ */
+object Interpolator:
+
+  def replace(line: String, replacements: Map[String, String]): String =
+    if replacements.isEmpty then return line
+
+    // Build all substitution pairs, sorted by key length descending
+    // so longer placeholders are replaced first.
+    val pairs = replacements.toVector
+      .flatMap { (key, value) =>
+        val k = key.stripPrefix(":")
+        Vector(
+          s":${ucfirst(k)}" -> ucfirst(value),
+          s":${k.toUpperCase}" -> value.toUpperCase.nn,
+          s":$k" -> value,
+        )
+      }
+      .sortBy(-_._1.length)
+
+    var result = line
+    for (placeholder, value) <- pairs do
+      result = result.replace(placeholder, value)
+    result
+
+  private def ucfirst(s: String): String =
+    if s.isEmpty then s
+    else s.charAt(0).toUpper.toString + s.substring(1)
diff --git a/src/main/scala/summer/phrasebook/KeyParser.scala b/src/main/scala/summer/phrasebook/KeyParser.scala
new file mode 100644
index 0000000..d1553f0
--- /dev/null
+++ b/src/main/scala/summer/phrasebook/KeyParser.scala
@@ -0,0 +1,52 @@
+package summer.phrasebook
+
+import java.util.concurrent.ConcurrentHashMap
+
+/**
+ * Parses translation key strings into [[ParsedKey]] components.
+ *
+ * Ported from Laravel's `Illuminate\Support\NamespacedItemResolver`.
+ *
+ * Format: `[namespace::]group[.item[.subitem...]]`
+ *
+ * The result is cached in a ConcurrentHashMap for repeated lookups.
+ */
+object KeyParser:
+  private val cache = new ConcurrentHashMap[String, ParsedKey]()
+
+  def parse(key: String): ParsedKey =
+    val cached = cache.get(key)
+    if cached != null then cached
+    else
+      val parsed = doParse(key)
+      cache.put(key, parsed)
+      parsed
+
+  def clearCache(): Unit = cache.clear()
+
+  private def doParse(key: String): ParsedKey =
+    val nsIdx = key.indexOf("::")
+    if nsIdx < 0 then
+      parseBasic(key)
+    else
+      parseNamespaced(key, nsIdx)
+
+  private def parseBasic(key: String): ParsedKey =
+    val dotIdx = key.indexOf('.')
+    if dotIdx < 0 then
+      ParsedKey(namespace = None, group = key, item = None)
+    else
+      val group = key.substring(0, dotIdx)
+      val item  = key.substring(dotIdx + 1)
+      ParsedKey(namespace = None, group = group, item = Some(item))
+
+  private def parseNamespaced(key: String, nsIdx: Int): ParsedKey =
+    val namespace = key.substring(0, nsIdx).toLowerCase.nn
+    val rest      = key.substring(nsIdx + 2)
+    val dotIdx    = rest.indexOf('.')
+    if dotIdx < 0 then
+      ParsedKey(namespace = Some(namespace), group = rest, item = None)
+    else
+      val group = rest.substring(0, dotIdx)
+      val item  = rest.substring(dotIdx + 1)
+      ParsedKey(namespace = Some(namespace), group = group, item = Some(item))
diff --git a/src/main/scala/summer/phrasebook/LocaleTag.scala b/src/main/scala/summer/phrasebook/LocaleTag.scala
new file mode 100644
index 0000000..6b75462
--- /dev/null
+++ b/src/main/scala/summer/phrasebook/LocaleTag.scala
@@ -0,0 +1,35 @@
+package summer.phrasebook
+
+/** Opaque type wrapping a locale identifier string (e.g. "en", "pl", "de-at"). */
+opaque type LocaleTag = String
+
+object LocaleTag:
+  def apply(raw: String): LocaleTag =
+    raw.trim.toLowerCase.nn
+
+  extension (tag: LocaleTag)
+    def value: String = tag
+
+    /** Parent locale: "de-at" -> Some("de"), "de" -> None */
+    def parent: Option[LocaleTag] =
+      val idx = tag.lastIndexOf('-')
+      if idx > 0 then Some(LocaleTag(tag.substring(0, idx))) else None
+
+    /** Language portion only: "de-at" -> "de", "zh-tw" -> "zh" */
+    def language: String =
+      val idx = tag.indexOf('-')
+      if idx > 0 then tag.substring(0, idx) else tag
+
+    /**
+     * Normalize for plural rule lookup: "zh-tw" -> "zh_TW".
+     * MessageSelector in Laravel uses underscore + uppercase region.
+     */
+    def toPluralLocale: String =
+      val idx = tag.indexOf('-')
+      if idx > 0 then
+        val lang   = tag.substring(0, idx)
+        val region = tag.substring(idx + 1).toUpperCase.nn
+        s"${lang}_${region}"
+      else tag
+
+  given CanEqual[LocaleTag, LocaleTag] = CanEqual.derived
diff --git a/src/main/scala/summer/phrasebook/PhrasebookTranslator.scala b/src/main/scala/summer/phrasebook/PhrasebookTranslator.scala
new file mode 100644
index 0000000..5e5abb9
--- /dev/null
+++ b/src/main/scala/summer/phrasebook/PhrasebookTranslator.scala
@@ -0,0 +1,201 @@
+package summer.phrasebook
+
+import java.nio.file.Path
+import java.util.concurrent.ConcurrentHashMap
+
+/**
+ * Full implementation of the [[Translator]] trait.
+ *
+ * Ported from Laravel's `Illuminate\Translation\Translator` +
+ * WinterCMS `Winter\Storm\Translation\Translator`.
+ *
+ * Thread-safe: uses ConcurrentHashMap for loaded translations and overrides.
+ */
+final class PhrasebookTranslator(
+    initialLocale: LocaleTag,
+    initialFallback: Option[LocaleTag] = None,
+) extends Translator:
+
+  // --- Mutable state ---
+
+  @volatile private var currentLocale: LocaleTag    = initialLocale
+  @volatile private var fallbackLocale: Option[LocaleTag] = initialFallback
+
+  /** namespace -> filesystem path */
+  private val namespaces = new ConcurrentHashMap[String, Path]()
+
+  /** alias -> real namespace */
+  private val aliases = new ConcurrentHashMap[String, String]()
+
+  /**
+   * Loaded translations: (namespace, group, locale) -> flat Map.
+   * Using a composite string key "ns|group|locale" for simplicity.
+   */
+  private val loaded = new ConcurrentHashMap[String, Map[String, String]]()
+
+  /** Runtime overrides: (locale, fullKey) -> value */
+  private val overrides = new ConcurrentHashMap[String, String]()
+
+  // --- Public API ---
+
+  override def get(
+      key: String,
+      replace: Map[String, String],
+      locale: Option[LocaleTag],
+  ): String =
+    val effectiveLocale = locale.getOrElse(currentLocale)
+    val parsed          = KeyParser.parse(key)
+    val ns              = resolveNamespace(parsed.namespace)
+
+    // Check runtime overrides first
+    val overrideKey = s"${effectiveLocale.value}|$key"
+    val overridden  = overrides.get(overrideKey)
+    if overridden != null then
+      return Interpolator.replace(overridden, replace)
+
+    // WinterCMS compat: validation.* without namespace -> system::validation.*
+    val (finalNs, finalParsed) =
+      if ns.isEmpty && parsed.group == "validation" then
+        (Some("system"), parsed.copy(namespace = Some("system")))
+      else
+        (ns, parsed)
+
+    // Walk fallback chain
+    val chain = FallbackChain.build(effectiveLocale, fallbackLocale)
+    val result = chain.iterator.flatMap { loc =>
+      getLine(finalNs, finalParsed.group, loc, finalParsed.item)
+    }.nextOption()
+
+    result match
+      case Some(line) => Interpolator.replace(line, replace)
+      case None       => key // Return raw key if not found (Laravel behavior)
+
+  override def choice(
+      key: String,
+      number: Int,
+      replace: Map[String, String],
+      locale: Option[LocaleTag],
+  ): String =
+    val effectiveLocale = locale.getOrElse(currentLocale)
+    val line            = get(key, Map.empty, Some(effectiveLocale))
+
+    // If get() returned the raw key (not found), return it with replacements
+    if line == key then
+      return Interpolator.replace(key, replace + ("count" -> number.toString))
+
+    val chosen = PluralRules.choose(line, number, effectiveLocale)
+    Interpolator.replace(chosen, replace + ("count" -> number.toString))
+
+  override def has(key: String, locale: Option[LocaleTag]): Boolean =
+    val effectiveLocale = locale.getOrElse(currentLocale)
+    val result          = get(key, Map.empty, Some(effectiveLocale))
+    result != key
+
+  override def allForLocale(
+      locale: LocaleTag,
+      namespace: Option[String],
+  ): Map[String, String] =
+    import scala.jdk.CollectionConverters.*
+    val prefix = namespace match
+      case Some(ns) => s"${resolveNamespace(Some(ns)).getOrElse(ns)}|"
+      case None     => ""
+
+    val result = Map.newBuilder[String, String]
+    for entry <- loaded.entrySet().asScala do
+      val compositeKey = entry.getKey.nn
+      if compositeKey.endsWith(s"|${locale.value}") &&
+        (prefix.isEmpty || compositeKey.startsWith(prefix))
+      then
+        val translations = entry.getValue.nn
+        result ++= translations
+    result.result()
+
+  override def setLocale(locale: LocaleTag): Unit =
+    require(!locale.value.contains("/") && !locale.value.contains("\\"),
+      s"Invalid locale: ${locale.value}")
+    currentLocale = locale
+
+  override def getLocale: LocaleTag = currentLocale
+
+  override def getFallback: Option[LocaleTag] = fallbackLocale
+
+  override def setFallback(locale: LocaleTag): Unit =
+    fallbackLocale = Some(locale)
+
+  override def addNamespace(namespace: String, path: Path): Unit =
+    namespaces.put(namespace.toLowerCase.nn, path)
+
+  override def addAlias(alias: String, namespace: String): Unit =
+    aliases.put(alias.toLowerCase.nn, namespace.toLowerCase.nn)
+
+  override def set(key: String, value: String, locale: Option[LocaleTag]): Unit =
+    val effectiveLocale = locale.getOrElse(currentLocale)
+    overrides.put(s"${effectiveLocale.value}|$key", value)
+
+  override def clearCache(): Unit =
+    loaded.clear()
+    overrides.clear()
+    KeyParser.clearCache()
+
+  // --- Internal ---
+
+  /**
+   * Resolve a namespace through aliases.
+   * Returns None for no-namespace keys.
+   */
+  private def resolveNamespace(ns: Option[String]): Option[String] =
+    ns.map { n =>
+      val lower   = n.toLowerCase.nn
+      val aliased = aliases.get(lower)
+      if aliased != null then aliased else lower
+    }
+
+  /**
+   * Get a translation line for a specific (namespace, group, locale, item) combo.
+   */
+  private def getLine(
+      namespace: Option[String],
+      group: String,
+      locale: LocaleTag,
+      item: Option[String],
+  ): Option[String] =
+    val translations = loadGroup(namespace, group, locale)
+    item match
+      case Some(itemKey) => translations.get(itemKey)
+      case None          => None // Requesting a whole group returns None for get()
+
+  /**
+   * Load translations for a (namespace, group, locale) triple.
+   * Lazy-loads from disk on first access.
+   */
+  private def loadGroup(
+      namespace: Option[String],
+      group: String,
+      locale: LocaleTag,
+  ): Map[String, String] =
+    val cacheKey = s"${namespace.getOrElse("*")}|$group|${locale.value}"
+    val cached   = loaded.get(cacheKey)
+    if cached != null then cached
+    else
+      val translations = doLoad(namespace, group, locale)
+      loaded.put(cacheKey, translations)
+      translations
+
+  /**
+   * Actually load translations from disk.
+   */
+  private def doLoad(
+      namespace: Option[String],
+      group: String,
+      locale: LocaleTag,
+  ): Map[String, String] =
+    namespace match
+      case None =>
+        // No namespace - would need a default path. Return empty.
+        Map.empty
+      case Some(ns) =>
+        val basePath = namespaces.get(ns)
+        if basePath == null then Map.empty
+        else
+          val confFile = basePath.resolve(locale.value).resolve(s"$group.conf")
+          HoconLoader.load(confFile)
diff --git a/src/main/scala/summer/phrasebook/PluralRules.scala b/src/main/scala/summer/phrasebook/PluralRules.scala
new file mode 100644
index 0000000..47398d8
--- /dev/null
+++ b/src/main/scala/summer/phrasebook/PluralRules.scala
@@ -0,0 +1,182 @@
+package summer.phrasebook
+
+/**
+ * CLDR-based plural rule engine ported from Laravel's `MessageSelector`.
+ *
+ * Two-pass approach:
+ *   1. Try explicit conditions: `{0} none|{1} one|[2,*] many`
+ *   2. Fall back to CLDR plural index for the locale.
+ *
+ * The `getPluralIndex` method covers ~40 locale groups from the
+ * Zend Framework / Laravel plural rules (BSD licensed).
+ */
+object PluralRules:
+
+  /**
+   * Choose the correct plural form from a pipe-delimited string.
+   *
+   * @param line   pipe-delimited translation e.g. "item|items" or "{0} none|{1} one|[2,*] many"
+   * @param number the count to pluralize for
+   * @param locale the locale tag (used for CLDR rules)
+   * @return the selected form string
+   */
+  def choose(line: String, number: Int, locale: LocaleTag): String =
+    val segments = line.split("\\|", -1).map(_.trim.nn).toVector
+
+    // Pass 1: try explicit conditions
+    extractExplicit(segments, number) match
+      case Some(value) => value
+      case None =>
+        // Pass 2: strip conditions, use CLDR index
+        val stripped = segments.map(stripCondition)
+        val index    = getPluralIndex(locale.toPluralLocale, number)
+        if index < stripped.size then stripped(index)
+        else stripped.lastOption.getOrElse(line)
+
+  /**
+   * Try to match a segment with explicit condition syntax.
+   * `{N}` = exact value, `[from,to]` = range (inclusive), `*` = wildcard.
+   */
+  private def extractExplicit(segments: Vector[String], number: Int): Option[String] =
+    segments.iterator.flatMap(extractFromString(_, number)).nextOption()
+
+  private val conditionPattern = """^[\{\[]([^\[\]\{\}]*)[\}\]](.*)""".r
+
+  private def extractFromString(part: String, number: Int): Option[String] =
+    part match
+      case conditionPattern(condition, value) =>
+        if condition.contains(',') then
+          val parts = condition.split(',')
+          if parts.length == 2 then
+            val from = parts(0).trim
+            val to   = parts(1).trim
+            val fromOk = from == "*" || number >= from.toInt
+            val toOk   = to == "*" || number <= to.toInt
+            if fromOk && toOk then Some(value.trim.nn) else None
+          else None
+        else
+          // Exact match
+          if condition.trim == number.toString then Some(value.trim.nn) else None
+      case _ => None
+
+  /** Strip the leading `{...}` or `[...]` condition prefix from a segment. */
+  private val stripPattern = """^[\{\[]([^\[\]\{\}]*)[\}\]](.*)""".r
+
+  private def stripCondition(segment: String): String =
+    segment match
+      case stripPattern(_, value) => value.trim.nn
+      case _                      => segment
+
+  /**
+   * Returns the 0-based plural form index for a given locale and number.
+   *
+   * Ported from Laravel `MessageSelector::getPluralIndex()` which itself
+   * derives from the Zend Framework (BSD license).
+   */
+  def getPluralIndex(locale: String, number: Int): Int =
+    // Use language portion for matching (e.g. "pt_BR" -> "pt")
+    val lang = locale.indexOf('_') match
+      case -1  => locale.toLowerCase.nn
+      case idx => locale.substring(0, idx).toLowerCase.nn
+
+    val n = math.abs(number)
+
+    lang match
+      // No plural: always 0
+      case "az" | "bo" | "dz" | "id" | "ja" | "jv" | "ka" | "km" |
+           "kn" | "ko" | "ms" | "th" | "tr" | "vi" | "zh" =>
+        0
+
+      // 2 forms: singular | plural (n == 1 ? 0 : 1)
+      case "af" | "bn" | "bg" | "ca" | "da" | "de" | "el" | "en" |
+           "eo" | "es" | "et" | "eu" | "fa" | "fi" | "fo" | "fur" |
+           "fy" | "gl" | "gu" | "ha" | "he" | "hu" | "is" | "it" |
+           "ku" | "lb" | "ml" | "mn" | "mr" | "nah" | "nb" | "ne" |
+           "nl" | "nn" | "no" | "om" | "or" | "pa" | "pap" | "ps" |
+           "pt" | "so" | "sq" | "sv" | "sw" | "ta" | "te" | "tk" |
+           "ur" | "zu" =>
+        if n == 1 then 0 else 1
+
+      // French-style: 0 and 1 are singular
+      case "am" | "bh" | "fil" | "fr" | "gun" | "hi" | "hy" |
+           "ln" | "mg" | "nso" | "ti" | "wa" | "xbr" =>
+        if n == 0 || n == 1 then 0 else 1
+
+      // Slavic (Russian/Ukrainian/Bosnian/Croatian/Serbian/Belarusian): 3 forms
+      case "be" | "bs" | "hr" | "ru" | "sr" | "uk" =>
+        if n % 10 == 1 && n % 100 != 11 then 0
+        else if n % 10 >= 2 && n % 10 <= 4 && (n % 100 < 10 || n % 100 >= 20) then 1
+        else 2
+
+      // Czech / Slovak: 3 forms
+      case "cs" | "sk" =>
+        if n == 1 then 0
+        else if n >= 2 && n <= 4 then 1
+        else 2
+
+      // Irish
+      case "ga" =>
+        if n == 1 then 0
+        else if n == 2 then 1
+        else 2
+
+      // Lithuanian: 3 forms
+      case "lt" =>
+        if n % 10 == 1 && n % 100 != 11 then 0
+        else if n % 10 >= 2 && (n % 100 < 10 || n % 100 >= 20) then 1
+        else 2
+
+      // Slovenian: 4 forms
+      case "sl" =>
+        if n % 100 == 1 then 0
+        else if n % 100 == 2 then 1
+        else if n % 100 == 3 || n % 100 == 4 then 2
+        else 3
+
+      // Macedonian: last digit
+      case "mk" =>
+        if n % 10 == 1 then 0 else 1
+
+      // Maltese: 4 forms
+      case "mt" =>
+        if n == 1 then 0
+        else if n == 0 || (n % 100 > 1 && n % 100 < 11) then 1
+        else if n % 100 > 10 && n % 100 < 20 then 2
+        else 3
+
+      // Latvian: 3 forms
+      case "lv" =>
+        if n == 0 then 0
+        else if n % 10 == 1 && n % 100 != 11 then 1
+        else 2
+
+      // Polish: 3 forms
+      case "pl" =>
+        if n == 1 then 0
+        else if n % 10 >= 2 && n % 10 <= 4 && (n % 100 < 12 || n % 100 > 14) then 1
+        else 2
+
+      // Welsh: 4 forms
+      case "cy" =>
+        if n == 1 then 0
+        else if n == 2 then 1
+        else if n == 8 || n == 11 then 2
+        else 3
+
+      // Romanian: 3 forms
+      case "ro" =>
+        if n == 1 then 0
+        else if n == 0 || (n % 100 > 0 && n % 100 < 20) then 1
+        else 2
+
+      // Arabic: 6 forms
+      case "ar" =>
+        if n == 0 then 0
+        else if n == 1 then 1
+        else if n == 2 then 2
+        else if n % 100 >= 3 && n % 100 <= 10 then 3
+        else if n % 100 >= 11 && n % 100 <= 99 then 4
+        else 5
+
+      // Default: no plural
+      case _ => 0
diff --git a/src/main/scala/summer/phrasebook/TranslationKey.scala b/src/main/scala/summer/phrasebook/TranslationKey.scala
new file mode 100644
index 0000000..3078abe
--- /dev/null
+++ b/src/main/scala/summer/phrasebook/TranslationKey.scala
@@ -0,0 +1,15 @@
+package summer.phrasebook
+
+/**
+ * Parsed translation key components.
+ *
+ * Examples:
+ *   "validation.required"            -> ParsedKey(None, "validation", Some("required"))
+ *   "winter.pages::lang.editor.title" -> ParsedKey(Some("winter.pages"), "lang", Some("editor.title"))
+ *   "lang"                           -> ParsedKey(None, "lang", None)
+ */
+final case class ParsedKey(
+    namespace: Option[String],
+    group: String,
+    item: Option[String],
+)
diff --git a/src/main/scala/summer/phrasebook/Translator.scala b/src/main/scala/summer/phrasebook/Translator.scala
new file mode 100644
index 0000000..b54204b
--- /dev/null
+++ b/src/main/scala/summer/phrasebook/Translator.scala
@@ -0,0 +1,66 @@
+package summer.phrasebook
+
+import java.nio.file.Path
+
+/**
+ * Public API for the translation engine.
+ *
+ * This trait mirrors Laravel's `Illuminate\Contracts\Translation\Translator`
+ * extended with WinterCMS additions (namespace aliases, runtime set, etc.).
+ */
+trait Translator:
+
+  /** Get the translation for the given key. Returns the key itself if not found. */
+  def get(
+      key: String,
+      replace: Map[String, String] = Map.empty,
+      locale: Option[LocaleTag] = None,
+  ): String
+
+  /**
+   * Get a translation with plural form selection.
+   *
+   * @param key     translation key (may contain pipe-delimited plural forms)
+   * @param number  the count for plural selection
+   * @param replace parameter replacements (`:count` is auto-added)
+   * @param locale  optional locale override
+   */
+  def choice(
+      key: String,
+      number: Int,
+      replace: Map[String, String] = Map.empty,
+      locale: Option[LocaleTag] = None,
+  ): String
+
+  /** Check whether a translation exists for the given key. */
+  def has(key: String, locale: Option[LocaleTag] = None): Boolean
+
+  /** Get all translations for a locale, optionally filtered by namespace. */
+  def allForLocale(
+      locale: LocaleTag,
+      namespace: Option[String] = None,
+  ): Map[String, String]
+
+  /** Set the current default locale. */
+  def setLocale(locale: LocaleTag): Unit
+
+  /** Get the current default locale. */
+  def getLocale: LocaleTag
+
+  /** Get the configured fallback locale. */
+  def getFallback: Option[LocaleTag]
+
+  /** Set the fallback locale. */
+  def setFallback(locale: LocaleTag): Unit
+
+  /** Register a namespace with its filesystem path for translation loading. */
+  def addNamespace(namespace: String, path: Path): Unit
+
+  /** Register a namespace alias (e.g. "user" -> "golem15.user"). */
+  def addAlias(alias: String, namespace: String): Unit
+
+  /** Set a runtime translation override. */
+  def set(key: String, value: String, locale: Option[LocaleTag] = None): Unit
+
+  /** Clear all cached/loaded translations. */
+  def clearCache(): Unit
diff --git a/src/test/resources/lang/ar/lang.conf b/src/test/resources/lang/ar/lang.conf
new file mode 100644
index 0000000..1f3ecb1
--- /dev/null
+++ b/src/test/resources/lang/ar/lang.conf
@@ -0,0 +1,3 @@
+# Arabic translations - 6 plural forms
+# zero|one|two|few(3-10)|many(11-99)|other(100+)
+posts_count = "لا مقالات|مقالة واحدة|مقالتان|:count مقالات|:count مقالة|:count مقالة"
diff --git a/src/test/resources/lang/de/lang.conf b/src/test/resources/lang/de/lang.conf
new file mode 100644
index 0000000..9dbb9a6
--- /dev/null
+++ b/src/test/resources/lang/de/lang.conf
@@ -0,0 +1,13 @@
+# German translations for test plugin
+labels {
+  pluginName = "Blog"
+  posts = "Beiträge"
+}
+
+post {
+  create_title = "Beitrag erstellen"
+}
+
+posts_count = ":count Beitrag|:count Beiträge"
+
+welcome = "Willkommen, :name!"
diff --git a/src/test/resources/lang/en/lang.conf b/src/test/resources/lang/en/lang.conf
new file mode 100644
index 0000000..cb7729e
--- /dev/null
+++ b/src/test/resources/lang/en/lang.conf
@@ -0,0 +1,23 @@
+# English translations for test plugin
+labels {
+  pluginName = "Blog"
+  posts = "Posts"
+  categories = "Categories"
+}
+
+post {
+  create_title = "Create Post"
+  edit_title = "Edit Post"
+  delete_confirm = "Are you sure you want to delete this post?"
+}
+
+# Pluralization: English has 2 forms (singular|plural)
+posts_count = ":count post|:count posts"
+
+# Explicit range syntax
+items_range = "{0} No items|{1} One item|[2,5] A few items|[6,*] Many items"
+
+# Parameter interpolation
+welcome = "Welcome, :name!"
+greeting = "Hello :Name, you have :count new messages"
+shout = "ATTENTION :NAME!"
diff --git a/src/test/resources/lang/en/validation.conf b/src/test/resources/lang/en/validation.conf
new file mode 100644
index 0000000..1ecf3ff
--- /dev/null
+++ b/src/test/resources/lang/en/validation.conf
@@ -0,0 +1,4 @@
+# English validation messages
+required = "The :attribute field is required."
+min_string = "The :attribute must be at least :min characters."
+email = "The :attribute must be a valid email address."
diff --git a/src/test/resources/lang/pl/lang.conf b/src/test/resources/lang/pl/lang.conf
new file mode 100644
index 0000000..9097cd5
--- /dev/null
+++ b/src/test/resources/lang/pl/lang.conf
@@ -0,0 +1,18 @@
+# Polish translations for test plugin
+labels {
+  pluginName = "Blog"
+  posts = "Wpisy"
+  categories = "Kategorie"
+}
+
+post {
+  create_title = "Utwórz wpis"
+  edit_title = "Edytuj wpis"
+  delete_confirm = "Czy na pewno chcesz usunąć ten wpis?"
+}
+
+# Polish: 3 CLDR forms (one|few|many)
+# n==1: wpis; n%10 in 2..4 && n%100 not in 12..14: wpisy; else: wpisów
+posts_count = ":count wpis|:count wpisy|:count wpisów"
+
+welcome = "Witaj, :name!"
diff --git a/src/test/resources/overrides/en/lang.conf b/src/test/resources/overrides/en/lang.conf
new file mode 100644
index 0000000..430ff57
--- /dev/null
+++ b/src/test/resources/overrides/en/lang.conf
@@ -0,0 +1,4 @@
+# App-level override for the test plugin
+labels {
+  pluginName = "My Custom Blog"
+}
diff --git a/src/test/scala/summer/phrasebook/FallbackChainSpec.scala b/src/test/scala/summer/phrasebook/FallbackChainSpec.scala
new file mode 100644
index 0000000..c4c1b6f
--- /dev/null
+++ b/src/test/scala/summer/phrasebook/FallbackChainSpec.scala
@@ -0,0 +1,64 @@
+package summer.phrasebook
+
+class FallbackChainSpec extends munit.FunSuite:
+
+  test("simple locale with no fallback") {
+    val chain = FallbackChain.build(LocaleTag("en"))
+    assertEquals(chain.map(_.value), Vector("en"))
+  }
+
+  test("locale with parent") {
+    val chain = FallbackChain.build(LocaleTag("de-at"))
+    assertEquals(chain.map(_.value), Vector("de-at", "de", "en"))
+  }
+
+  test("locale with configured fallback") {
+    val chain = FallbackChain.build(
+      LocaleTag("fr"),
+      fallback = Some(LocaleTag("en")),
+    )
+    assertEquals(chain.map(_.value), Vector("fr", "en"))
+  }
+
+  test("locale with parent and different fallback") {
+    val chain = FallbackChain.build(
+      LocaleTag("de-at"),
+      fallback = Some(LocaleTag("fr")),
+    )
+    assertEquals(chain.map(_.value), Vector("de-at", "de", "fr", "en"))
+  }
+
+  test("no duplicates when fallback matches parent") {
+    val chain = FallbackChain.build(
+      LocaleTag("de-at"),
+      fallback = Some(LocaleTag("de")),
+    )
+    assertEquals(chain.map(_.value), Vector("de-at", "de", "en"))
+  }
+
+  test("no duplicates when requested is core") {
+    val chain = FallbackChain.build(LocaleTag("en"))
+    assertEquals(chain.map(_.value), Vector("en"))
+  }
+
+  test("no duplicates when fallback is core") {
+    val chain = FallbackChain.build(
+      LocaleTag("pl"),
+      fallback = Some(LocaleTag("en")),
+    )
+    assertEquals(chain.map(_.value), Vector("pl", "en"))
+  }
+
+  test("multi-level parent chain") {
+    // Hypothetical deeply nested locale
+    val chain = FallbackChain.build(LocaleTag("zh-hans-cn"))
+    assertEquals(chain.map(_.value), Vector("zh-hans-cn", "zh-hans", "zh", "en"))
+  }
+
+  test("custom core locale") {
+    val chain = FallbackChain.build(
+      LocaleTag("fr"),
+      core = LocaleTag("de"),
+    )
+    assertEquals(chain.map(_.value), Vector("fr", "de"))
+  }
diff --git a/src/test/scala/summer/phrasebook/HoconLoaderSpec.scala b/src/test/scala/summer/phrasebook/HoconLoaderSpec.scala
new file mode 100644
index 0000000..32d9dae
--- /dev/null
+++ b/src/test/scala/summer/phrasebook/HoconLoaderSpec.scala
@@ -0,0 +1,49 @@
+package summer.phrasebook
+
+import java.nio.file.Path
+
+class HoconLoaderSpec extends munit.FunSuite:
+
+  val testResources: Path =
+    Path.of(getClass.getClassLoader.getResource("lang/en/lang.conf").toURI).getParent.getParent
+
+  test("load English lang.conf") {
+    val translations = HoconLoader.load(testResources.resolve("en/lang.conf"))
+    assertEquals(translations.get("labels.pluginName"), Some("Blog"))
+    assertEquals(translations.get("labels.posts"), Some("Posts"))
+    assertEquals(translations.get("post.create_title"), Some("Create Post"))
+  }
+
+  test("load Polish lang.conf") {
+    val translations = HoconLoader.load(testResources.resolve("pl/lang.conf"))
+    assertEquals(translations.get("labels.pluginName"), Some("Blog"))
+    assertEquals(translations.get("labels.posts"), Some("Wpisy"))
+    assertEquals(translations.get("post.create_title"), Some("Utwórz wpis"))
+  }
+
+  test("nested keys are dot-separated") {
+    val translations = HoconLoader.load(testResources.resolve("en/lang.conf"))
+    assertEquals(translations.get("post.edit_title"), Some("Edit Post"))
+    assertEquals(translations.get("post.delete_confirm"), Some("Are you sure you want to delete this post?"))
+  }
+
+  test("translation with placeholders is loaded as-is") {
+    val translations = HoconLoader.load(testResources.resolve("en/lang.conf"))
+    assertEquals(translations.get("welcome"), Some("Welcome, :name!"))
+  }
+
+  test("plural string with pipes is loaded as-is") {
+    val translations = HoconLoader.load(testResources.resolve("en/lang.conf"))
+    assertEquals(translations.get("posts_count"), Some(":count post|:count posts"))
+  }
+
+  test("nonexistent file returns empty map") {
+    val translations = HoconLoader.load(testResources.resolve("xx/nonexistent.conf"))
+    assert(translations.isEmpty)
+  }
+
+  test("validation.conf loads correctly") {
+    val translations = HoconLoader.load(testResources.resolve("en/validation.conf"))
+    assertEquals(translations.get("required"), Some("The :attribute field is required."))
+    assertEquals(translations.get("email"), Some("The :attribute must be a valid email address."))
+  }
diff --git a/src/test/scala/summer/phrasebook/InterpolatorSpec.scala b/src/test/scala/summer/phrasebook/InterpolatorSpec.scala
new file mode 100644
index 0000000..0eba629
--- /dev/null
+++ b/src/test/scala/summer/phrasebook/InterpolatorSpec.scala
@@ -0,0 +1,63 @@
+package summer.phrasebook
+
+class InterpolatorSpec extends munit.FunSuite:
+
+  test("simple replacement") {
+    val result = Interpolator.replace("Hello, :name!", Map("name" -> "World"))
+    assertEquals(result, "Hello, World!")
+  }
+
+  test("ucfirst replacement") {
+    val result = Interpolator.replace("Hello, :Name!", Map("name" -> "world"))
+    assertEquals(result, "Hello, World!")
+  }
+
+  test("uppercase replacement") {
+    val result = Interpolator.replace("ATTENTION :NAME!", Map("name" -> "world"))
+    assertEquals(result, "ATTENTION WORLD!")
+  }
+
+  test("all three case variants in one string") {
+    val result = Interpolator.replace(
+      ":name :Name :NAME",
+      Map("name" -> "hello"),
+    )
+    assertEquals(result, "hello Hello HELLO")
+  }
+
+  test("multiple different replacements") {
+    val result = Interpolator.replace(
+      "The :attribute must be at least :min characters.",
+      Map("attribute" -> "password", "min" -> "8"),
+    )
+    assertEquals(result, "The password must be at least 8 characters.")
+  }
+
+  test("empty replacements returns original") {
+    val result = Interpolator.replace("No replacements here", Map.empty)
+    assertEquals(result, "No replacements here")
+  }
+
+  test("key with colon prefix is stripped") {
+    // Users can pass ":name" or "name" - both should work
+    val result = Interpolator.replace("Hello :name", Map(":name" -> "World"))
+    assertEquals(result, "Hello World")
+  }
+
+  test("no placeholder in string") {
+    val result = Interpolator.replace("No placeholders", Map("name" -> "World"))
+    assertEquals(result, "No placeholders")
+  }
+
+  test("longer keys replaced first to avoid partial match") {
+    val result = Interpolator.replace(
+      ":count_total and :count items",
+      Map("count" -> "5", "count_total" -> "10"),
+    )
+    assertEquals(result, "10 and 5 items")
+  }
+
+  test("empty value replacement") {
+    val result = Interpolator.replace("Hello :name!", Map("name" -> ""))
+    assertEquals(result, "Hello !")
+  }
diff --git a/src/test/scala/summer/phrasebook/KeyParserSpec.scala b/src/test/scala/summer/phrasebook/KeyParserSpec.scala
new file mode 100644
index 0000000..ed29845
--- /dev/null
+++ b/src/test/scala/summer/phrasebook/KeyParserSpec.scala
@@ -0,0 +1,67 @@
+package summer.phrasebook
+
+class KeyParserSpec extends munit.FunSuite:
+
+  override def beforeEach(context: BeforeEach): Unit =
+    KeyParser.clearCache()
+
+  test("parse simple group.item key") {
+    val parsed = KeyParser.parse("validation.required")
+    assertEquals(parsed.namespace, None)
+    assertEquals(parsed.group, "validation")
+    assertEquals(parsed.item, Some("required"))
+  }
+
+  test("parse group-only key (no item)") {
+    val parsed = KeyParser.parse("lang")
+    assertEquals(parsed.namespace, None)
+    assertEquals(parsed.group, "lang")
+    assertEquals(parsed.item, None)
+  }
+
+  test("parse namespaced key") {
+    val parsed = KeyParser.parse("winter.pages::lang.editor.title")
+    assertEquals(parsed.namespace, Some("winter.pages"))
+    assertEquals(parsed.group, "lang")
+    assertEquals(parsed.item, Some("editor.title"))
+  }
+
+  test("parse namespaced key with simple item") {
+    val parsed = KeyParser.parse("system::validation.required")
+    assertEquals(parsed.namespace, Some("system"))
+    assertEquals(parsed.group, "validation")
+    assertEquals(parsed.item, Some("required"))
+  }
+
+  test("parse namespaced group-only key") {
+    val parsed = KeyParser.parse("system::validation")
+    assertEquals(parsed.namespace, Some("system"))
+    assertEquals(parsed.group, "validation")
+    assertEquals(parsed.item, None)
+  }
+
+  test("namespace is lowercased") {
+    val parsed = KeyParser.parse("Winter.Pages::lang.title")
+    assertEquals(parsed.namespace, Some("winter.pages"))
+  }
+
+  test("deeply nested item preserves dots") {
+    val parsed = KeyParser.parse("system::validation.custom.email.required")
+    assertEquals(parsed.namespace, Some("system"))
+    assertEquals(parsed.group, "validation")
+    assertEquals(parsed.item, Some("custom.email.required"))
+  }
+
+  test("results are cached") {
+    val first  = KeyParser.parse("test.key")
+    val second = KeyParser.parse("test.key")
+    assert(first eq second, "Cached results should be the same reference")
+  }
+
+  test("clearCache removes cached entries") {
+    val first = KeyParser.parse("cached.key")
+    KeyParser.clearCache()
+    val second = KeyParser.parse("cached.key")
+    assertEquals(first, second)
+    assert(!(first eq second), "After clear, should be a new instance")
+  }
diff --git a/src/test/scala/summer/phrasebook/LocaleTagSpec.scala b/src/test/scala/summer/phrasebook/LocaleTagSpec.scala
new file mode 100644
index 0000000..9bbbb94
--- /dev/null
+++ b/src/test/scala/summer/phrasebook/LocaleTagSpec.scala
@@ -0,0 +1,34 @@
+package summer.phrasebook
+
+class LocaleTagSpec extends munit.FunSuite:
+
+  test("apply normalizes to lowercase") {
+    assertEquals(LocaleTag("EN").value, "en")
+    assertEquals(LocaleTag("De-AT").value, "de-at")
+  }
+
+  test("apply trims whitespace") {
+    assertEquals(LocaleTag("  en  ").value, "en")
+  }
+
+  test("parent returns None for simple locale") {
+    assertEquals(LocaleTag("en").parent, None)
+  }
+
+  test("parent returns parent locale") {
+    assertEquals(LocaleTag("de-at").parent.map(_.value), Some("de"))
+  }
+
+  test("language returns language portion") {
+    assertEquals(LocaleTag("de-at").language, "de")
+    assertEquals(LocaleTag("en").language, "en")
+  }
+
+  test("toPluralLocale converts dash to underscore+uppercase region") {
+    assertEquals(LocaleTag("zh-tw").toPluralLocale, "zh_TW")
+    assertEquals(LocaleTag("pt-br").toPluralLocale, "pt_BR")
+  }
+
+  test("toPluralLocale leaves simple locale unchanged") {
+    assertEquals(LocaleTag("en").toPluralLocale, "en")
+  }
diff --git a/src/test/scala/summer/phrasebook/PhrasebookTranslatorSpec.scala b/src/test/scala/summer/phrasebook/PhrasebookTranslatorSpec.scala
new file mode 100644
index 0000000..53eed15
--- /dev/null
+++ b/src/test/scala/summer/phrasebook/PhrasebookTranslatorSpec.scala
@@ -0,0 +1,296 @@
+package summer.phrasebook
+
+import java.nio.file.Path
+
+class PhrasebookTranslatorSpec extends munit.FunSuite:
+
+  val langDir: Path =
+    Path.of(getClass.getClassLoader.getResource("lang/en/lang.conf").toURI).getParent.getParent
+
+  val overridesDir: Path =
+    Path.of(getClass.getClassLoader.getResource("overrides/en/lang.conf").toURI).getParent.getParent
+
+  def makeTranslator(): PhrasebookTranslator =
+    val t = new PhrasebookTranslator(
+      initialLocale = LocaleTag("en"),
+      initialFallback = Some(LocaleTag("en")),
+    )
+    t.addNamespace("test.plugin", langDir)
+    t
+
+  override def beforeEach(context: BeforeEach): Unit =
+    KeyParser.clearCache()
+
+  // --- Basic get ---
+
+  test("get simple translation") {
+    val t = makeTranslator()
+    assertEquals(
+      t.get("test.plugin::lang.labels.pluginName"),
+      "Blog",
+    )
+  }
+
+  test("get nested translation") {
+    val t = makeTranslator()
+    assertEquals(
+      t.get("test.plugin::lang.post.create_title"),
+      "Create Post",
+    )
+  }
+
+  test("get returns raw key when not found") {
+    val t = makeTranslator()
+    assertEquals(
+      t.get("test.plugin::lang.nonexistent.key"),
+      "test.plugin::lang.nonexistent.key",
+    )
+  }
+
+  test("get from different group (validation)") {
+    val t = makeTranslator()
+    assertEquals(
+      t.get("test.plugin::validation.required"),
+      "The :attribute field is required.",
+    )
+  }
+
+  // --- Parameter replacement ---
+
+  test("get with replacements") {
+    val t = makeTranslator()
+    assertEquals(
+      t.get("test.plugin::lang.welcome", Map("name" -> "Alice")),
+      "Welcome, Alice!",
+    )
+  }
+
+  test("get with multiple replacements and case variants") {
+    val t = makeTranslator()
+    assertEquals(
+      t.get("test.plugin::lang.greeting", Map("name" -> "alice", "count" -> "3")),
+      "Hello Alice, you have 3 new messages",
+    )
+  }
+
+  test("get with uppercase replacement") {
+    val t = makeTranslator()
+    assertEquals(
+      t.get("test.plugin::lang.shout", Map("name" -> "alice")),
+      "ATTENTION ALICE!",
+    )
+  }
+
+  test("validation message with attribute replacement") {
+    val t = makeTranslator()
+    assertEquals(
+      t.get("test.plugin::validation.required", Map("attribute" -> "email")),
+      "The email field is required.",
+    )
+  }
+
+  // --- Locale switching ---
+
+  test("get translation in Polish") {
+    val t = makeTranslator()
+    assertEquals(
+      t.get("test.plugin::lang.labels.posts", locale = Some(LocaleTag("pl"))),
+      "Wpisy",
+    )
+  }
+
+  test("get translation in German") {
+    val t = makeTranslator()
+    assertEquals(
+      t.get("test.plugin::lang.labels.posts", locale = Some(LocaleTag("de"))),
+      "Beiträge",
+    )
+  }
+
+  test("setLocale changes default locale") {
+    val t = makeTranslator()
+    t.setLocale(LocaleTag("pl"))
+    assertEquals(
+      t.get("test.plugin::lang.post.create_title"),
+      "Utwórz wpis",
+    )
+  }
+
+  // --- Fallback ---
+
+  test("falls back to English when key missing in Polish") {
+    val t = makeTranslator()
+    // Polish has no "shout" key, should fall back to English
+    assertEquals(
+      t.get("test.plugin::lang.shout", Map("name" -> "alice"), locale = Some(LocaleTag("pl"))),
+      "ATTENTION ALICE!",
+    )
+  }
+
+  test("falls back to English when key missing in German") {
+    val t = makeTranslator()
+    assertEquals(
+      t.get("test.plugin::lang.post.edit_title", locale = Some(LocaleTag("de"))),
+      "Edit Post",
+    )
+  }
+
+  // --- Pluralization via choice ---
+
+  test("choice English singular") {
+    val t = makeTranslator()
+    assertEquals(
+      t.choice("test.plugin::lang.posts_count", 1),
+      "1 post",
+    )
+  }
+
+  test("choice English plural") {
+    val t = makeTranslator()
+    assertEquals(
+      t.choice("test.plugin::lang.posts_count", 5),
+      "5 posts",
+    )
+  }
+
+  test("choice Polish forms") {
+    val t = makeTranslator()
+    // 1 wpis (one)
+    assertEquals(
+      t.choice("test.plugin::lang.posts_count", 1, locale = Some(LocaleTag("pl"))),
+      "1 wpis",
+    )
+    // 2 wpisy (few)
+    assertEquals(
+      t.choice("test.plugin::lang.posts_count", 2, locale = Some(LocaleTag("pl"))),
+      "2 wpisy",
+    )
+    // 5 wpisów (many)
+    assertEquals(
+      t.choice("test.plugin::lang.posts_count", 5, locale = Some(LocaleTag("pl"))),
+      "5 wpisów",
+    )
+    // 22 wpisy (few: n%10=2, n%100=22)
+    assertEquals(
+      t.choice("test.plugin::lang.posts_count", 22, locale = Some(LocaleTag("pl"))),
+      "22 wpisy",
+    )
+  }
+
+  test("choice with explicit range syntax") {
+    val t = makeTranslator()
+    assertEquals(
+      t.choice("test.plugin::lang.items_range", 0),
+      "No items",
+    )
+    assertEquals(
+      t.choice("test.plugin::lang.items_range", 1),
+      "One item",
+    )
+    assertEquals(
+      t.choice("test.plugin::lang.items_range", 3),
+      "A few items",
+    )
+    assertEquals(
+      t.choice("test.plugin::lang.items_range", 10),
+      "Many items",
+    )
+  }
+
+  // --- has ---
+
+  test("has returns true for existing key") {
+    val t = makeTranslator()
+    assert(t.has("test.plugin::lang.labels.pluginName"))
+  }
+
+  test("has returns false for missing key") {
+    val t = makeTranslator()
+    assert(!t.has("test.plugin::lang.nonexistent"))
+  }
+
+  // --- Namespace alias ---
+
+  test("namespace alias resolves correctly") {
+    val t = makeTranslator()
+    t.addAlias("blog", "test.plugin")
+    assertEquals(
+      t.get("blog::lang.labels.pluginName"),
+      "Blog",
+    )
+  }
+
+  // --- Runtime set ---
+
+  test("set overrides a translation at runtime") {
+    val t = makeTranslator()
+    t.set("test.plugin::lang.labels.pluginName", "My Blog")
+    assertEquals(
+      t.get("test.plugin::lang.labels.pluginName"),
+      "My Blog",
+    )
+  }
+
+  test("set override is locale-specific") {
+    val t = makeTranslator()
+    t.set("test.plugin::lang.labels.pluginName", "My Blog", Some(LocaleTag("en")))
+    // Polish should still return original
+    assertEquals(
+      t.get("test.plugin::lang.labels.pluginName", locale = Some(LocaleTag("pl"))),
+      "Blog",
+    )
+  }
+
+  // --- clearCache ---
+
+  test("clearCache resets loaded translations") {
+    val t = makeTranslator()
+    // Load something
+    t.get("test.plugin::lang.labels.pluginName")
+    // Clear
+    t.clearCache()
+    // Should still work (reloads)
+    assertEquals(
+      t.get("test.plugin::lang.labels.pluginName"),
+      "Blog",
+    )
+  }
+
+  // --- setLocale validation ---
+
+  test("setLocale rejects path traversal") {
+    val t = makeTranslator()
+    intercept[IllegalArgumentException] {
+      t.setLocale(LocaleTag("../etc/passwd"))
+    }
+  }
+
+  // --- getLocale / getFallback ---
+
+  test("getLocale returns current locale") {
+    val t = makeTranslator()
+    assertEquals(t.getLocale.value, "en")
+    t.setLocale(LocaleTag("pl"))
+    assertEquals(t.getLocale.value, "pl")
+  }
+
+  test("getFallback returns configured fallback") {
+    val t = makeTranslator()
+    assertEquals(t.getFallback.map(_.value), Some("en"))
+  }
+
+  test("setFallback changes fallback locale") {
+    val t = makeTranslator()
+    t.setFallback(LocaleTag("de"))
+    assertEquals(t.getFallback.map(_.value), Some("de"))
+  }
+
+  // --- Unknown namespace ---
+
+  test("unknown namespace returns raw key") {
+    val t = makeTranslator()
+    assertEquals(
+      t.get("unknown.ns::lang.key"),
+      "unknown.ns::lang.key",
+    )
+  }
diff --git a/src/test/scala/summer/phrasebook/PluralRulesSpec.scala b/src/test/scala/summer/phrasebook/PluralRulesSpec.scala
new file mode 100644
index 0000000..b892f4a
--- /dev/null
+++ b/src/test/scala/summer/phrasebook/PluralRulesSpec.scala
@@ -0,0 +1,213 @@
+package summer.phrasebook
+
+class PluralRulesSpec extends munit.FunSuite:
+
+  // --- getPluralIndex tests ---
+
+  test("English: 2 forms (singular/plural)") {
+    assertEquals(PluralRules.getPluralIndex("en", 0), 1)
+    assertEquals(PluralRules.getPluralIndex("en", 1), 0)
+    assertEquals(PluralRules.getPluralIndex("en", 2), 1)
+    assertEquals(PluralRules.getPluralIndex("en", 100), 1)
+  }
+
+  test("French: 0 and 1 are singular") {
+    assertEquals(PluralRules.getPluralIndex("fr", 0), 0)
+    assertEquals(PluralRules.getPluralIndex("fr", 1), 0)
+    assertEquals(PluralRules.getPluralIndex("fr", 2), 1)
+    assertEquals(PluralRules.getPluralIndex("fr", 5), 1)
+  }
+
+  test("Polish: 3 forms (one/few/many)") {
+    assertEquals(PluralRules.getPluralIndex("pl", 1), 0)   // one
+    assertEquals(PluralRules.getPluralIndex("pl", 2), 1)   // few
+    assertEquals(PluralRules.getPluralIndex("pl", 3), 1)   // few
+    assertEquals(PluralRules.getPluralIndex("pl", 4), 1)   // few
+    assertEquals(PluralRules.getPluralIndex("pl", 5), 2)   // many
+    assertEquals(PluralRules.getPluralIndex("pl", 10), 2)  // many
+    assertEquals(PluralRules.getPluralIndex("pl", 11), 2)  // many
+    assertEquals(PluralRules.getPluralIndex("pl", 12), 2)  // many (12-14 are exceptions)
+    assertEquals(PluralRules.getPluralIndex("pl", 13), 2)  // many
+    assertEquals(PluralRules.getPluralIndex("pl", 14), 2)  // many
+    assertEquals(PluralRules.getPluralIndex("pl", 22), 1)  // few
+    assertEquals(PluralRules.getPluralIndex("pl", 23), 1)  // few
+    assertEquals(PluralRules.getPluralIndex("pl", 24), 1)  // few
+    assertEquals(PluralRules.getPluralIndex("pl", 25), 2)  // many
+    assertEquals(PluralRules.getPluralIndex("pl", 100), 2) // many
+    assertEquals(PluralRules.getPluralIndex("pl", 112), 2) // many (n%100=12, exception)
+  }
+
+  test("Russian: 3 forms (Slavic rule)") {
+    assertEquals(PluralRules.getPluralIndex("ru", 1), 0)   // one
+    assertEquals(PluralRules.getPluralIndex("ru", 2), 1)   // few
+    assertEquals(PluralRules.getPluralIndex("ru", 5), 2)   // many
+    assertEquals(PluralRules.getPluralIndex("ru", 11), 2)  // many (exception: 11-19)
+    assertEquals(PluralRules.getPluralIndex("ru", 21), 0)  // one
+    assertEquals(PluralRules.getPluralIndex("ru", 22), 1)  // few
+    assertEquals(PluralRules.getPluralIndex("ru", 25), 2)  // many
+    assertEquals(PluralRules.getPluralIndex("ru", 111), 2) // many (n%100=11)
+    assertEquals(PluralRules.getPluralIndex("ru", 121), 0) // one (n%10=1, n%100=21)
+  }
+
+  test("Czech: 3 forms") {
+    assertEquals(PluralRules.getPluralIndex("cs", 1), 0)
+    assertEquals(PluralRules.getPluralIndex("cs", 2), 1)
+    assertEquals(PluralRules.getPluralIndex("cs", 4), 1)
+    assertEquals(PluralRules.getPluralIndex("cs", 5), 2)
+    assertEquals(PluralRules.getPluralIndex("cs", 100), 2)
+  }
+
+  test("Japanese: no plural (always 0)") {
+    assertEquals(PluralRules.getPluralIndex("ja", 0), 0)
+    assertEquals(PluralRules.getPluralIndex("ja", 1), 0)
+    assertEquals(PluralRules.getPluralIndex("ja", 100), 0)
+  }
+
+  test("Chinese: no plural (always 0)") {
+    assertEquals(PluralRules.getPluralIndex("zh", 0), 0)
+    assertEquals(PluralRules.getPluralIndex("zh", 1), 0)
+    assertEquals(PluralRules.getPluralIndex("zh", 42), 0)
+  }
+
+  test("Arabic: 6 forms") {
+    assertEquals(PluralRules.getPluralIndex("ar", 0), 0)    // zero
+    assertEquals(PluralRules.getPluralIndex("ar", 1), 1)    // one
+    assertEquals(PluralRules.getPluralIndex("ar", 2), 2)    // two
+    assertEquals(PluralRules.getPluralIndex("ar", 3), 3)    // few (3-10)
+    assertEquals(PluralRules.getPluralIndex("ar", 10), 3)   // few
+    assertEquals(PluralRules.getPluralIndex("ar", 11), 4)   // many (11-99)
+    assertEquals(PluralRules.getPluralIndex("ar", 99), 4)   // many
+    assertEquals(PluralRules.getPluralIndex("ar", 100), 5)  // other (100+)
+    assertEquals(PluralRules.getPluralIndex("ar", 1000), 5) // other
+  }
+
+  test("Slovenian: 4 forms") {
+    assertEquals(PluralRules.getPluralIndex("sl", 1), 0)   // n%100==1
+    assertEquals(PluralRules.getPluralIndex("sl", 2), 1)   // n%100==2
+    assertEquals(PluralRules.getPluralIndex("sl", 3), 2)   // n%100==3 or 4
+    assertEquals(PluralRules.getPluralIndex("sl", 4), 2)
+    assertEquals(PluralRules.getPluralIndex("sl", 5), 3)   // other
+    assertEquals(PluralRules.getPluralIndex("sl", 101), 0) // n%100==1
+    assertEquals(PluralRules.getPluralIndex("sl", 102), 1) // n%100==2
+  }
+
+  test("Welsh: 4 forms") {
+    assertEquals(PluralRules.getPluralIndex("cy", 1), 0)
+    assertEquals(PluralRules.getPluralIndex("cy", 2), 1)
+    assertEquals(PluralRules.getPluralIndex("cy", 8), 2)
+    assertEquals(PluralRules.getPluralIndex("cy", 11), 2)
+    assertEquals(PluralRules.getPluralIndex("cy", 5), 3)
+  }
+
+  test("Maltese: 4 forms") {
+    assertEquals(PluralRules.getPluralIndex("mt", 1), 0)   // one
+    assertEquals(PluralRules.getPluralIndex("mt", 0), 1)   // few (0, or n%100 2-10)
+    assertEquals(PluralRules.getPluralIndex("mt", 2), 1)   // few
+    assertEquals(PluralRules.getPluralIndex("mt", 10), 1)  // few
+    assertEquals(PluralRules.getPluralIndex("mt", 11), 2)  // many (n%100 11-19)
+    assertEquals(PluralRules.getPluralIndex("mt", 19), 2)  // many
+    assertEquals(PluralRules.getPluralIndex("mt", 20), 3)  // other
+  }
+
+  test("Lithuanian: 3 forms") {
+    assertEquals(PluralRules.getPluralIndex("lt", 1), 0)   // one (n%10==1, not 11)
+    assertEquals(PluralRules.getPluralIndex("lt", 2), 1)   // few (n%10>=2, n%100<10 or >=20)
+    assertEquals(PluralRules.getPluralIndex("lt", 10), 2)  // other
+    assertEquals(PluralRules.getPluralIndex("lt", 11), 2)  // exception
+    assertEquals(PluralRules.getPluralIndex("lt", 21), 0)  // one
+    assertEquals(PluralRules.getPluralIndex("lt", 12), 2)  // exception (n%100 in 10-19)
+  }
+
+  test("Latvian: 3 forms") {
+    assertEquals(PluralRules.getPluralIndex("lv", 0), 0)   // zero
+    assertEquals(PluralRules.getPluralIndex("lv", 1), 1)   // one (n%10==1, not 11)
+    assertEquals(PluralRules.getPluralIndex("lv", 2), 2)   // other
+    assertEquals(PluralRules.getPluralIndex("lv", 11), 2)  // other (exception)
+    assertEquals(PluralRules.getPluralIndex("lv", 21), 1)  // one
+  }
+
+  test("Romanian: 3 forms") {
+    assertEquals(PluralRules.getPluralIndex("ro", 1), 0)   // one
+    assertEquals(PluralRules.getPluralIndex("ro", 0), 1)   // few (0 or n%100 1-19)
+    assertEquals(PluralRules.getPluralIndex("ro", 19), 1)  // few
+    assertEquals(PluralRules.getPluralIndex("ro", 20), 2)  // other
+    assertEquals(PluralRules.getPluralIndex("ro", 100), 2) // other
+  }
+
+  test("Macedonian: last digit rule") {
+    assertEquals(PluralRules.getPluralIndex("mk", 1), 0)   // n%10==1
+    assertEquals(PluralRules.getPluralIndex("mk", 2), 1)
+    assertEquals(PluralRules.getPluralIndex("mk", 11), 0)  // n%10==1
+    assertEquals(PluralRules.getPluralIndex("mk", 21), 0)  // n%10==1
+  }
+
+  test("Irish: 3 forms") {
+    assertEquals(PluralRules.getPluralIndex("ga", 1), 0)
+    assertEquals(PluralRules.getPluralIndex("ga", 2), 1)
+    assertEquals(PluralRules.getPluralIndex("ga", 3), 2)
+  }
+
+  test("locale with region uses language portion") {
+    // pt_BR should use "pt" rules (2 forms)
+    assertEquals(PluralRules.getPluralIndex("pt_BR", 1), 0)
+    assertEquals(PluralRules.getPluralIndex("pt_BR", 2), 1)
+  }
+
+  test("unknown locale returns 0") {
+    assertEquals(PluralRules.getPluralIndex("xx", 5), 0)
+  }
+
+  test("negative numbers use absolute value") {
+    assertEquals(PluralRules.getPluralIndex("en", -1), 0)
+    assertEquals(PluralRules.getPluralIndex("en", -5), 1)
+  }
+
+  // --- choose() tests ---
+
+  test("choose with simple English plural") {
+    val en = LocaleTag("en")
+    assertEquals(PluralRules.choose("item|items", 1, en), "item")
+    assertEquals(PluralRules.choose("item|items", 0, en), "items")
+    assertEquals(PluralRules.choose("item|items", 5, en), "items")
+  }
+
+  test("choose with explicit conditions") {
+    val en = LocaleTag("en")
+    assertEquals(PluralRules.choose("{0} No items|{1} One item|[2,*] Many items", 0, en), "No items")
+    assertEquals(PluralRules.choose("{0} No items|{1} One item|[2,*] Many items", 1, en), "One item")
+    assertEquals(PluralRules.choose("{0} No items|{1} One item|[2,*] Many items", 5, en), "Many items")
+    assertEquals(PluralRules.choose("{0} No items|{1} One item|[2,*] Many items", 100, en), "Many items")
+  }
+
+  test("choose with explicit range") {
+    val en = LocaleTag("en")
+    assertEquals(PluralRules.choose("[1,3] Few|[4,*] Many", 2, en), "Few")
+    assertEquals(PluralRules.choose("[1,3] Few|[4,*] Many", 4, en), "Many")
+  }
+
+  test("choose with Polish 3 forms") {
+    val pl = LocaleTag("pl")
+    val line = "wpis|wpisy|wpisów"
+    assertEquals(PluralRules.choose(line, 1, pl), "wpis")
+    assertEquals(PluralRules.choose(line, 2, pl), "wpisy")
+    assertEquals(PluralRules.choose(line, 5, pl), "wpisów")
+    assertEquals(PluralRules.choose(line, 22, pl), "wpisy")
+    assertEquals(PluralRules.choose(line, 12, pl), "wpisów")
+  }
+
+  test("choose with Arabic 6 forms") {
+    val ar = LocaleTag("ar")
+    val line = "zero|one|two|few|many|other"
+    assertEquals(PluralRules.choose(line, 0, ar), "zero")
+    assertEquals(PluralRules.choose(line, 1, ar), "one")
+    assertEquals(PluralRules.choose(line, 2, ar), "two")
+    assertEquals(PluralRules.choose(line, 5, ar), "few")
+    assertEquals(PluralRules.choose(line, 50, ar), "many")
+    assertEquals(PluralRules.choose(line, 100, ar), "other")
+  }
+
+  test("choose falls back to last segment if index out of bounds") {
+    val en = LocaleTag("en")
+    // Only 1 segment but English wants index 1 for n!=1
+    assertEquals(PluralRules.choose("only one form", 5, en), "only one form")
+  }