diff --git a/README.md b/README.md
index fbd61bd..7829f60 100644
--- a/README.md
+++ b/README.md
@@ -53,7 +53,7 @@ It owns the `users` table extensions, `user_throttle`, `jwt_blacklist`, `user_ap
   ```
 
   Boot compiles the catalog from the activated plugins (`classes.CompileAPITokenScopes`, published as `*classes.APITokenScopeCatalog` and as `pact.MemberTokenScopeCatalog`) and fails on a name that does not pass `pact.ValidTokenScopeName`, a built-in redeclared, a reserved name `backend` or `offline_access`, a name two plugins declare (the error names both), an empty label or description, an `Implies` other than empty, `read` or `write` (the error names the plugin and that value), or a label or description phrase key (containing `::`) that the translator does not have. `Implies` is only those three values: empty means the declared scope is not implied, and the built-ins `read`, `write` and `ai` keep it empty. Implied names are not stored on a token row or a fingerprint. `POST /tokens` and `classes.MintAPITokenFor(app, …)` accept the scopes of the application's own catalog (`classes.APITokenScopesFor(app)`) and answer an undeclared one with the usual 422 `The selected scope is invalid.`; the console's `user:token:create` uses `MintAPITokenFor` too. Applications mint with `MintAPITokenFor`. The older `classes.MintAPIToken` is deprecated; it keeps its signature and accepts the built-ins plus every scope a catalog compiled in the same process declared, a fallback for existing callers; production runs one application per process, so that is the application's own catalog, while a test binary that boots several applications shares it (`classes.ResetDeclaredAPITokenScopes` clears it, for tests only). `user.scope:<name>`, `middleware.Scope` and `APIToken.HasScope` check a declared scope like a built-in, and `GET /tokens` lists it in `scopes`. `GET /tokens/scopes` (`jwt.auth`) answers `{"data":[{"name","label","description","built_in"}]}`, the built-ins first, then the declared scopes in activation order, with phrase-key labels and descriptions translated for the request locale. The admin `g15_` token catalog never shows these scopes (it reads `pact.HasTokenScopes` only). `classes.AllowedAPITokenScopes` stays the built-in list.
-- **Mail templates.** Activation, reset, reactivation and invitation mails in Polish and English (`golem15.user::mail.*`, layout `golem15.user::mail.layouts.user`); the locale follows the user (`ResolveMailLocale`). The invitation (`golem15.user::mail.invite`) is sent when an administrator creates a user with "Send invitation by email" set; it carries the user's login and an activation link, never the password.
+- **Mail templates.** Activation, reset, reactivation and invitation mails in Polish and English (`golem15.user::mail.*`, layout `golem15.user::mail.layouts.user`); the Polish templates have Polish subjects. The language follows the user's `preferred_locale`, then the request locale (the translate plugin's, else the raw `Accept-Language` header), each normalised to `pl` or `en` by `NormalizeMailLocale` (comma lists, q-weights, region subtags such as `en-GB`, case), then `golem15.user.mail.fallback_locale` (`ResolveMailLocaleFor`). The invitation (`golem15.user::mail.invite`) is sent when an administrator creates a user with "Send invitation by email" set; it carries the user's login and an activation link, never the password.
 - **Extension events.** `RegisterEvent` fires after a user row is created; its `Payload` is a copy of the register input (extra keys a client sent included, `password` and `password_confirmation` never), so it is untrusted client input. `CreatedEvent` (`classes.CreatedEventName` `user.Created`) fires from API register and admin create. `/register` logs a failing `RegisterEvent` or `CreatedEvent` listener at Error level and continues: a committed registration is never undone or failed by a listener. `DeletingEvent` (`classes.DeletingEventName` `user.Deleting`) fires from `FormAfterDelete` before `ForceDeleteCleanup` / unscoped delete, inside the same transaction. `GetApiArrayEvent` lets other plugins merge keys into the user payload.
 - **Registration for other plugins.** `RegisterUser`, `FireRegisterEvent`, `IssueToken` and `APIArray` are the steps `POST /register` runs, exported so another plugin (a first-run setup endpoint, for example) creates a user with the same validation, hashing, event, token and payload.
 - **OAuth providers.** `GET /oauth-providers` lists the social sign-in providers a social-login plugin has enabled, read on every request from its published `classes.OAuthProviderSource`. Without one it answers the empty list, as before.
@@ -257,6 +257,7 @@ Packages:
   - `LoginEvent` / `LoginEventName` (see Login event).
   - `DeviceShortCodeAlphabet`, `NewDeviceToken()`, `NewDeviceShortCode()`, `NormalizeDeviceShortCode(s)`, `DeviceAuthTTL(app)`, `DeviceAuthURLFor(app, token, shortCode)`, `DeviceAuthURL(app, token)` (the same with the token in place of the short code), `DeviceDisplayName(stored, userAgent)`, `RevokeDeviceSessions(ctx, db, userIDs...)` (revokes the users' pending and confirmed sessions; the plugin calls it for every `AccessRevokedEvent`), `FollowDeviceJTI(ctx, db, oldJTI, newJTI)` (moves a used session to a refreshed token's jti and reports a session revoked meanwhile; `POST /refresh` calls it and refuses the refresh then) and `HandOverDeviceJTI(ctx, db, oldJTI, newJTI)` (the same move without the report), the device authorization helpers; device tokens are hashed with `HashAPIToken`.
   - `TwoFactorAdmin` (`HasTwoFactor(ctx, tx, userID)`, `Reset(ctx, tx, user)`) and `PermissionResetTwoFactor`, the Reset 2FA slot (see Admin: Reset 2FA), and `(*AccessRevokedEvent).PasswordChanged()` (see Password-change hook).
+  - `MailTemplate(base, locale)`, `ResolveMailLocale(ctx, user)`, `ResolveMailLocaleFor(ctx, app, user)`, `NormalizeMailLocale(raw)` and `MailFallbackLocale(app)`, the mail language helpers. `ResolveMailLocale` returns `pl`, `en` or `""` (neither the user nor the request names a supported language); a plugin that mails in the user's language calls `ResolveMailLocaleFor`, which adds the application's fallback and always returns `pl` or `en`, and passes its result to `MailTemplate`.
   - `HandleGenerator`, `AssignHandle(ctx, db, user)`, `CreatedEvent` / `CreatedEventName`, `DeletingEvent` / `DeletingEventName`, `DeactivatingEvent` / `DeactivatingEventName`, `AuthorizeUser` and `RegisterUserAuthorizer`.
 - `middleware`: `APITokenGuard`, `Scope`, `Permission` and `BearerToken`.
 - `models`: `User`, `Throttle`, `Organisation`, `UserGroup`, `UsersGroup` (the `users_groups` pivot), `APIToken`, `OAuthIdentity`, `FrontendPermission`, `BackendLink`, `BackendLinkRequest`, `DeviceAuthSession` (with `DeviceStatusPending`, `DeviceStatusConfirmed`, `DeviceStatusUsed`, `DeviceStatusExpired` and `DeviceStatusRevoked`) and `PermissionSet`. `PermissionSet` reads a stored permission column tolerantly (NULL, an empty string, `[]`, numeric strings) and writes a JSON object of integers; `ParsePermissionSet` and `PermissionSet.Text` do the same for a column that is a plain string on its model. `UserGroup.UsersCount` is filled only by a query that selects `users_count` (the admin list does). `Organisation.Members` is the relation behind the members manager, `Organisation.AttachRelations` declares its avatar (`OrganisationAvatarField`) and its logo (`OrganisationLogoField`), and `Slugify` is the slug rule. `User.FilterScope` answers the admin list scope `FilterByGroup`, and `User.AttachRelations` declares the avatar (`AvatarField`). `User.RegisterFillable` is the registration allow-list; `User.Fillable` is the wider mass-assignment backstop of the admin Users form (cabana intersects it with the form's fields) and of trusted application code.
@@ -280,6 +281,7 @@ Defaults live in `config/config.yaml` and are merged under `golem15.user.*`. Any
 | `golem15.user.activation.activate_mode` | `auto` | `auto`, `user` (mailed code) or `admin`. |
 | `golem15.user.activation.reset_ttl_minutes` | `60` | Password reset code lifetime. |
 | `golem15.user.activation.activation_ttl_hours` | `72` | Activation code lifetime. |
+| `golem15.user.mail.fallback_locale` | `pl` | The mail language when the user's `preferred_locale` and the request locale name neither `pl` nor `en`. `pl` keeps the unsuffixed templates; `en` (`SUMMER_GOLEM15__USER__MAIL__FALLBACK_LOCALE=en`) suits English-only sites. Region forms such as `en-GB` are accepted; any other value fails boot. Read on every send. The user and the request still come first, so a browser that asks for Polish first gets Polish mail on an application whose fallback is `en`. |
 | `golem15.user.registration.allow_registration` | `true` | Whether `POST /register` accepts new accounts. |
 | `golem15.user.registration.use_register_throttle` | `true` | Whether registration is limited to three accounts per IP an hour. |
 | `golem15.user.throttle.attempt_limit` | `5` | Failed logins before suspension. |
@@ -325,6 +327,8 @@ go test ./...           # full run, starts Postgres with testcontainers (needs D
 
 `TestRegisterCORSPath` reads the host application's `config/http.yaml`, so run the tests inside an application checkout where the plugin is mounted at `plugins/golem15/user`.
 
+The mail language is pinned by `TestNormalizeMailLocale`, `TestResolveMailLocale`, `TestResolveMailLocaleFor` and `TestMailFallbackLocale` (in `classes`, `-short`), `TestMailFallbackLocaleConfig`, `TestMailFallbackLocaleBoot` and `TestPolishMailSubjects` (root, `-short`; the last renders each Polish template next to its `-en` twin and requires a different subject) and `TestMailLocaleFromRequest` (Postgres, skipped under `-short`: `POST /forgot-password` with a raw `Accept-Language` request locale, the default and an `en` fallback, and a saved `en-GB`).
+
 `TestCommandsDocumented` (in `-short`) logs, without failing, each command whose help lacks a description, long help, examples or an option description (`bonfire.CheckDocs`).
 
 `TestRegisterIgnoresProtectedFields` posts organisation, IP and account-flag keys to `/register` and `RegisterUser` and checks that only the allow-listed columns and the server-side IP are stored; `TestRegisterListenerErrors` checks that failing register and created listeners do not fail the registration.
diff --git a/classes/mail.go b/classes/mail.go
index 43d6b84..c58fcae 100644
--- a/classes/mail.go
+++ b/classes/mail.go
@@ -2,27 +2,128 @@ package classes
 
 import (
 	"context"
+	"fmt"
+	"strconv"
+	"strings"
 
 	"git.golem15.com/golem15/sm-user-plugin/models"
+	"git.golem15.com/golem15/summercms/modules/backpack"
 	"git.golem15.com/golem15/summercms/modules/towel"
 )
 
-// MailTemplate appends -en for English and leaves every other locale,
-// including empty and pl, on the unsuffixed Polish template name.
+const (
+	// mailFallbackLocaleKey names the mail language used when neither the
+	// user nor the request names one the plugin has mail for.
+	mailFallbackLocaleKey = "golem15.user.mail.fallback_locale"
+	// mailLocalePL is the unsuffixed (Polish) template set and the default
+	// fallback.
+	mailLocalePL = "pl"
+	// mailLocaleEN is the -en template set.
+	mailLocaleEN = "en"
+)
+
+// supportedMailLocale reports whether the plugin has mail in lang.
+func supportedMailLocale(lang string) bool {
+	return lang == mailLocalePL || lang == mailLocaleEN
+}
+
+// NormalizeMailLocale reduces a locale value to the language the plugin has
+// mail for: pl, en or "". It accepts a plain code (en, PL), a region form
+// (en-GB, pl_PL) and an Accept-Language list with q-weights
+// (en-GB,en;q=0.9). Entries whose q does not parse, lies outside 0..1 or is
+// 0 are dropped, as are languages other than pl and en (the wildcard
+// included); of the rest the one with the highest q wins, ties going to the
+// earliest. The result never holds anything but pl, en or "".
+func NormalizeMailLocale(raw string) string {
+	best, bestQ := "", -1.0
+	for _, entry := range strings.Split(raw, ",") {
+		parts := strings.Split(entry, ";")
+		tag := strings.ToLower(strings.TrimSpace(parts[0]))
+		if i := strings.IndexAny(tag, "-_"); i >= 0 {
+			tag = tag[:i]
+		}
+		if !supportedMailLocale(tag) {
+			continue
+		}
+		q, ok := 1.0, true
+		for _, param := range parts[1:] {
+			param = strings.TrimSpace(param)
+			if len(param) < 2 || !strings.EqualFold(param[:2], "q=") {
+				continue
+			}
+			v, err := strconv.ParseFloat(strings.TrimSpace(param[2:]), 64)
+			if err != nil || v < 0 || v > 1 {
+				ok = false
+				break
+			}
+			q = v
+		}
+		if !ok || q == 0 {
+			continue
+		}
+		if q > bestQ {
+			best, bestQ = tag, q
+		}
+	}
+	return best
+}
+
+// MailTemplate appends -en when locale normalises to English
+// (NormalizeMailLocale) and leaves every other value, empty included, on the
+// unsuffixed Polish template name. Callers that want the application's
+// configured fallback resolve the locale with ResolveMailLocaleFor first.
 func MailTemplate(base, locale string) string {
-	if locale == "en" {
+	if NormalizeMailLocale(locale) == mailLocaleEN {
 		return base + "-en"
 	}
 	return base
 }
 
-// ResolveMailLocale prefers the user's saved locale, then the request locale.
+// ResolveMailLocale prefers the user's saved locale, then the request locale,
+// each normalised by NormalizeMailLocale; a value naming neither pl nor en is
+// skipped. It returns pl, en or "" (neither names a supported language).
 func ResolveMailLocale(ctx context.Context, user *models.User) string {
-	if user != nil && user.PreferredLocale != nil && *user.PreferredLocale != "" {
-		return *user.PreferredLocale
+	if user != nil && user.PreferredLocale != nil {
+		if loc := NormalizeMailLocale(*user.PreferredLocale); loc != "" {
+			return loc
+		}
 	}
-	if loc, ok := towel.Locale(ctx); ok {
-		return loc
+	if ctx != nil {
+		if raw, ok := towel.Locale(ctx); ok {
+			return NormalizeMailLocale(raw)
+		}
 	}
 	return ""
 }
+
+// MailFallbackLocale returns golem15.user.mail.fallback_locale normalised to
+// pl or en: the mail language when neither the user nor the request names
+// one the plugin has mail for. The default is pl (the unsuffixed
+// templates); an English-only site sets en, for example through
+// SUMMER_GOLEM15__USER__MAIL__FALLBACK_LOCALE=en. A missing app, config or
+// blank value gives pl; any other value gives pl and an error naming the key.
+func MailFallbackLocale(app *backpack.App) (string, error) {
+	if app == nil || app.Config == nil {
+		return mailLocalePL, nil
+	}
+	raw := strings.TrimSpace(app.Config.String(mailFallbackLocaleKey))
+	if raw == "" {
+		return mailLocalePL, nil
+	}
+	if loc := NormalizeMailLocale(raw); loc != "" {
+		return loc, nil
+	}
+	return mailLocalePL, fmt.Errorf("golem15.user: %s must be pl or en, got %q", mailFallbackLocaleKey, raw)
+}
+
+// ResolveMailLocaleFor returns the language to mail user in, always pl or
+// en: ResolveMailLocale, then the application's MailFallbackLocale. It is the
+// helper other plugins use to mail in the user's language with the
+// application's fallback; an invalid fallback (rejected at boot) counts as pl.
+func ResolveMailLocaleFor(ctx context.Context, app *backpack.App, user *models.User) string {
+	if loc := ResolveMailLocale(ctx, user); loc != "" {
+		return loc
+	}
+	loc, _ := MailFallbackLocale(app)
+	return loc
+}
diff --git a/classes/mail_test.go b/classes/mail_test.go
index b609f63..d6b9950 100644
--- a/classes/mail_test.go
+++ b/classes/mail_test.go
@@ -1,16 +1,187 @@
 package classes
 
-import "testing"
+import (
+	"context"
+	"strings"
+	"testing"
+
+	"git.golem15.com/golem15/sm-user-plugin/models"
+	"git.golem15.com/golem15/summercms/modules/backpack"
+	"git.golem15.com/golem15/summercms/modules/compass"
+	"git.golem15.com/golem15/summercms/modules/towel"
+)
 
 func TestMailTemplate(t *testing.T) {
 	base := "golem15.user::mail.reactivate"
-	if got := MailTemplate(base, "pl"); got != base {
-		t.Fatalf("pl = %s", got)
-	}
-	if got := MailTemplate(base, ""); got != base {
-		t.Fatalf("empty = %s", got)
-	}
-	if got := MailTemplate(base, "en"); got != base+"-en" {
-		t.Fatalf("en = %s", got)
+	for _, tc := range []struct {
+		locale, want string
+	}{
+		{"pl", base},
+		{"", base},
+		{"en", base + "-en"},
+		{"en-GB", base + "-en"},
+		{"en-GB,en;q=0.9", base + "-en"},
+		{"de", base},
+		{"pl-PL,en;q=0.5", base},
+	} {
+		if got := MailTemplate(base, tc.locale); got != tc.want {
+			t.Errorf("MailTemplate(%q) = %s, want %s", tc.locale, got, tc.want)
+		}
+	}
+}
+
+func TestNormalizeMailLocale(t *testing.T) {
+	for _, tc := range []struct {
+		raw, want string
+	}{
+		{"en", "en"},
+		{"pl", "pl"},
+		{"EN", "en"},
+		{"en-GB", "en"},
+		{"en_US", "en"},
+		{"pl-PL", "pl"},
+		{" en-gb ; q=0.9 ", "en"},
+		{"en-GB,en;q=0.9", "en"},
+		{"pl-PL,pl;q=0.9,en-US;q=0.8,en;q=0.7", "pl"},
+		{"de-DE,de;q=0.9,en;q=0.8", "en"},
+		{"de-DE,en;q=0.5,pl;q=0.7", "pl"},
+		{"en;q=0.5,pl;q=0.5", "en"},
+		{"en;q=0,pl", "pl"},
+		{"en;q=0", ""},
+		{"en;q=abc,pl;q=0.1", "pl"},
+		{"en;q=1.5", ""},
+		{"english", ""},
+		{"de", ""},
+		{"*", ""},
+		{"", ""},
+		{"   ", ""},
+	} {
+		if got := NormalizeMailLocale(tc.raw); got != tc.want {
+			t.Errorf("NormalizeMailLocale(%q) = %q, want %q", tc.raw, got, tc.want)
+		}
+	}
+}
+
+func TestResolveMailLocale(t *testing.T) {
+	str := func(s string) *string { return &s }
+	header := "en-GB,en;q=0.9"
+	for _, tc := range []struct {
+		name    string
+		saved   *string
+		nilUser bool
+		request string
+		want    string
+	}{
+		{"saved region form wins", str("en-GB"), false, "pl", "en"},
+		{"saved upper case", str("PL"), false, "", "pl"},
+		{"unsupported saved falls through", str("de"), false, header, "en"},
+		{"nil user, raw header", nil, true, header, "en"},
+		{"nil user, no request locale", nil, true, "", ""},
+		{"empty saved, unsupported request", str(""), false, "fr", ""},
+	} {
+		ctx := context.Background()
+		if tc.request != "" {
+			ctx = towel.WithLocale(ctx, tc.request)
+		}
+		var user *models.User
+		if !tc.nilUser {
+			user = &models.User{PreferredLocale: tc.saved}
+		}
+		if got := ResolveMailLocale(ctx, user); got != tc.want {
+			t.Errorf("%s: got %q, want %q", tc.name, got, tc.want)
+		}
+	}
+}
+
+func mailTestApp(t *testing.T, environ ...string) *backpack.App {
+	t.Helper()
+	cfg, err := compass.Open(compass.Options{Dir: t.TempDir(), Env: "development", Environ: append([]string{"SUMMER_ENV=development"}, environ...)})
+	if err != nil {
+		t.Fatal(err)
+	}
+	return backpack.New(cfg)
+}
+
+func TestMailFallbackLocale(t *testing.T) {
+	if got, err := MailFallbackLocale(nil); got != "pl" || err != nil {
+		t.Errorf("nil app: %q %v", got, err)
+	}
+	if got, err := MailFallbackLocale(&backpack.App{}); got != "pl" || err != nil {
+		t.Errorf("nil config: %q %v", got, err)
+	}
+	if got, err := MailFallbackLocale(mailTestApp(t)); got != "pl" || err != nil {
+		t.Errorf("no key: %q %v", got, err)
+	}
+	for _, tc := range []struct {
+		value, want string
+		bad         bool
+	}{
+		{"", "pl", false},
+		{"   ", "pl", false},
+		{"en", "en", false},
+		{"EN-gb", "en", false},
+		{"pl", "pl", false},
+		{"de", "pl", true},
+	} {
+		app := mailTestApp(t)
+		if err := app.Config.Set("golem15.user.mail.fallback_locale", tc.value); err != nil {
+			t.Fatal(err)
+		}
+		got, err := MailFallbackLocale(app)
+		if got != tc.want {
+			t.Errorf("%q: got %q, want %q", tc.value, got, tc.want)
+		}
+		if tc.bad {
+			if err == nil || !strings.Contains(err.Error(), "golem15.user.mail.fallback_locale") || !strings.Contains(err.Error(), `"de"`) {
+				t.Errorf("%q: error %v", tc.value, err)
+			}
+		} else if err != nil {
+			t.Errorf("%q: unexpected error %v", tc.value, err)
+		}
+	}
+	app := mailTestApp(t, "SUMMER_GOLEM15__USER__MAIL__FALLBACK_LOCALE=en")
+	if got, err := MailFallbackLocale(app); got != "en" || err != nil {
+		t.Errorf("env override: %q %v", got, err)
+	}
+}
+
+func TestResolveMailLocaleFor(t *testing.T) {
+	const restore = "golem15.user::mail.restore"
+	user := &models.User{}
+	headerCtx := towel.WithLocale(context.Background(), "en-GB,en;q=0.9")
+
+	// The reported bug: raw Accept-Language, no preferred_locale, default config.
+	if got := MailTemplate(restore, ResolveMailLocaleFor(headerCtx, mailTestApp(t), user)); got != restore+"-en" {
+		t.Errorf("raw header: %s", got)
+	}
+	if got := MailTemplate(restore, ResolveMailLocaleFor(context.Background(), mailTestApp(t), user)); got != restore {
+		t.Errorf("default fallback: %s", got)
+	}
+	if got := ResolveMailLocaleFor(context.Background(), nil, nil); got != "pl" {
+		t.Errorf("nil app and user: %q", got)
+	}
+
+	set := mailTestApp(t)
+	if err := set.Config.Set("golem15.user.mail.fallback_locale", "en"); err != nil {
+		t.Fatal(err)
+	}
+	if got := MailTemplate(restore, ResolveMailLocaleFor(context.Background(), set, user)); got != restore+"-en" {
+		t.Errorf("fallback en via Set: %s", got)
+	}
+	env := mailTestApp(t, "SUMMER_GOLEM15__USER__MAIL__FALLBACK_LOCALE=en")
+	if got := MailTemplate(restore, ResolveMailLocaleFor(context.Background(), env, user)); got != restore+"-en" {
+		t.Errorf("fallback en via env: %s", got)
+	}
+	// The user and the request still come first.
+	if got := ResolveMailLocaleFor(towel.WithLocale(context.Background(), "pl-PL,en;q=0.5"), set, user); got != "pl" {
+		t.Errorf("Polish-first browser on an en fallback: %q", got)
+	}
+
+	bad := mailTestApp(t)
+	if err := bad.Config.Set("golem15.user.mail.fallback_locale", "de"); err != nil {
+		t.Fatal(err)
+	}
+	if got := ResolveMailLocaleFor(context.Background(), bad, user); got != "pl" {
+		t.Errorf("invalid fallback: %q", got)
 	}
 }
diff --git a/config/config.yaml b/config/config.yaml
index 35b9c3d..0b319bb 100644
--- a/config/config.yaml
+++ b/config/config.yaml
@@ -15,6 +15,14 @@ activation:
   activate_mode: auto
   reset_ttl_minutes: 60
   activation_ttl_hours: 72
+mail:
+  # The language of this plugin's mails when neither the user's
+  # preferred_locale nor the request locale (the translate plugin's locale,
+  # else the Accept-Language header) names one the plugin has mail for (pl or
+  # en). pl keeps the unsuffixed Polish templates; an English-only site sets
+  # en (SUMMER_GOLEM15__USER__MAIL__FALLBACK_LOCALE=en). Any other value
+  # fails boot.
+  fallback_locale: pl
 registration:
   allow_registration: true
   use_register_throttle: true
diff --git a/controllers/api_controller.go b/controllers/api_controller.go
index 95c8e55..da3e053 100644
--- a/controllers/api_controller.go
+++ b/controllers/api_controller.go
@@ -160,7 +160,7 @@ func Login(app *backpack.App) http.HandlerFunc {
 				writeOpaque500(w)
 				return
 			}
-			sendTemplateVars(ctx, app, mailName(ctx, user, "golem15.user::mail.reactivate"), user.Email, mailVars(displayName(user), "", ""))
+			sendTemplateVars(ctx, app, mailName(ctx, app, user, "golem15.user::mail.reactivate"), user.Email, mailVars(displayName(user), "", ""))
 		}
 		// A failed last_seen write is logged and never fails the login.
 		if err := classes.TouchLastSeen(ctx, gdb, user.ID); err != nil {
@@ -476,7 +476,7 @@ func ForgotPassword(app *backpack.App) http.HandlerFunc {
 				pluginLog.Error("golem15.user: issue reset code", "err", err)
 			} else {
 				packed := strconv.FormatUint(uint64(user.ID), 10) + "!" + code
-				sendTemplateVars(ctx, app, mailName(ctx, user, "golem15.user::mail.restore"), user.Email, mailVars(displayName(user), resetLink(app, user.ID, code), packed))
+				sendTemplateVars(ctx, app, mailName(ctx, app, user, "golem15.user::mail.restore"), user.Email, mailVars(displayName(user), resetLink(app, user.ID, code), packed))
 			}
 		}
 		writeJSON(w, http.StatusOK, map[string]any{"message": forgotPasswordBody})
@@ -937,7 +937,7 @@ func Register(app *backpack.App) http.HandlerFunc {
 				packed := strconv.FormatUint(uint64(user.ID), 10) + "!" + code
 				vars = mailVars(displayName(user), activationLink(app, user.ID, code), packed)
 			}
-			sendTemplateVars(ctx, app, mailName(ctx, user, "golem15.user::mail.activate"), user.Email, vars)
+			sendTemplateVars(ctx, app, mailName(ctx, app, user, "golem15.user::mail.activate"), user.Email, vars)
 			writeJSON(w, http.StatusOK, map[string]any{"message": "Activation email sent"})
 			return
 		}
@@ -1157,8 +1157,8 @@ func blacklistGrace(app *backpack.App) time.Duration {
 	return time.Duration(secs) * time.Second
 }
 
-func mailName(ctx context.Context, user *models.User, base string) string {
-	return classes.MailTemplate(base, classes.ResolveMailLocale(ctx, user))
+func mailName(ctx context.Context, app *backpack.App, user *models.User, base string) string {
+	return classes.MailTemplate(base, classes.ResolveMailLocaleFor(ctx, app, user))
 }
 
 func sendTemplate(ctx context.Context, app *backpack.App, template, to string) {
diff --git a/controllers/registration.go b/controllers/registration.go
index 4bbc08b..15d3d51 100644
--- a/controllers/registration.go
+++ b/controllers/registration.go
@@ -118,7 +118,7 @@ func FinishLogin(ctx context.Context, app *backpack.App, db *gorm.DB, user *mode
 			return err
 		}
 		user.DeletedAt = gorm.DeletedAt{}
-		sendTemplateVars(ctx, app, mailName(ctx, user, "golem15.user::mail.reactivate"), user.Email, mailVars(displayName(user), "", ""))
+		sendTemplateVars(ctx, app, mailName(ctx, app, user, "golem15.user::mail.reactivate"), user.Email, mailVars(displayName(user), "", ""))
 	}
 	if ip != "" {
 		if err := db.WithContext(ctx).Unscoped().Model(user).Update("last_ip_address", ip).Error; err != nil {
diff --git a/controllers/users_admin_controller.go b/controllers/users_admin_controller.go
index 91525a5..4d14536 100644
--- a/controllers/users_admin_controller.go
+++ b/controllers/users_admin_controller.go
@@ -378,7 +378,7 @@ func (c usersAdminController) FormAfterCreate(ctx context.Context, model any) er
 	vars["login"], vars["Login"] = email, email
 	hours := c.configInt("golem15.user.activation.activation_ttl_hours", 72)
 	vars["hours"], vars["Hours"] = hours, hours
-	template := mailName(ctx, user, "golem15.user::mail.invite")
+	template := mailName(ctx, app, user, "golem15.user::mail.invite")
 	lagoon.AfterCommit(ctx, tx, func(ctx context.Context, _ *gorm.DB) {
 		sendTemplateVars(ctx, app, template, email, vars)
 	})
diff --git a/mail_locale_test.go b/mail_locale_test.go
new file mode 100644
index 0000000..e7fa70e
--- /dev/null
+++ b/mail_locale_test.go
@@ -0,0 +1,114 @@
+package user
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"git.golem15.com/golem15/sm-user-plugin/classes"
+	"git.golem15.com/golem15/sm-user-plugin/controllers"
+	"git.golem15.com/golem15/summercms/modules/backpack"
+	"git.golem15.com/golem15/summercms/modules/towel"
+)
+
+const mailFallbackKey = "golem15.user.mail.fallback_locale"
+
+// TestMailFallbackLocaleConfig reads golem15.user.mail.fallback_locale from
+// the plugin's config.yaml default and from overrides.
+func TestMailFallbackLocaleConfig(t *testing.T) {
+	app := bareApp(t)
+	if err := app.Config.MergePlugin("golem15.user", configFS); err != nil {
+		t.Fatal(err)
+	}
+	if v, ok := app.Config.Lookup(mailFallbackKey); !ok || v != "pl" {
+		t.Fatalf("config.yaml default = %#v (set %v), want pl", v, ok)
+	}
+	if got, err := classes.MailFallbackLocale(app); got != "pl" || err != nil {
+		t.Fatalf("default: %q %v", got, err)
+	}
+	for _, tc := range []struct{ value, want string }{{"en", "en"}, {"EN-gb", "en"}} {
+		if err := app.Config.Set(mailFallbackKey, tc.value); err != nil {
+			t.Fatal(err)
+		}
+		if got, err := classes.MailFallbackLocale(app); got != tc.want || err != nil {
+			t.Errorf("%q: %q %v", tc.value, got, err)
+		}
+	}
+	if err := app.Config.Set(mailFallbackKey, "de"); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := classes.MailFallbackLocale(app); err == nil || !strings.Contains(err.Error(), mailFallbackKey) {
+		t.Errorf("de: %v", err)
+	}
+}
+
+// TestMailFallbackLocaleBoot fails Boot on an unsupported fallback before
+// anything is started, and leaves an unset key alone.
+func TestMailFallbackLocaleBoot(t *testing.T) {
+	withSecret := func(t *testing.T) *backpack.App {
+		app := bareApp(t)
+		if err := app.Config.Set("golem15.user.jwt.secret", "mail-locale-boot-secret"); err != nil {
+			t.Fatal(err)
+		}
+		return app
+	}
+	app := withSecret(t)
+	if err := app.Config.Set(mailFallbackKey, "de"); err != nil {
+		t.Fatal(err)
+	}
+	if err := (&Plugin{}).Boot(app); err == nil || !strings.Contains(err.Error(), mailFallbackKey) {
+		t.Fatalf("Boot with fallback de: %v", err)
+	}
+	if err := (&Plugin{}).Boot(withSecret(t)); err != nil && strings.Contains(err.Error(), mailFallbackKey) {
+		t.Fatalf("Boot with the key unset: %v", err)
+	}
+}
+
+// TestMailLocaleFromRequest sends the reset mail through POST
+// /forgot-password with the request locale the router stores when no
+// translate plugin is installed: the raw Accept-Language header.
+func TestMailLocaleFromRequest(t *testing.T) {
+	if testing.Short() {
+		t.Skip("Postgres (testcontainers)")
+	}
+	app, gdb := sessionApp(t, "0s")
+	rec, _ := publishMemoryMail(t, app)
+
+	forgot := func(t *testing.T, email, locale string) {
+		t.Helper()
+		req := httptest.NewRequest(http.MethodPost, "/_user/api/v1/forgot-password", strings.NewReader(`{"email":"`+email+`"}`))
+		req.Host = "example.com"
+		req.Header.Set("Content-Type", "application/json")
+		if locale != "" {
+			req = req.WithContext(towel.WithLocale(req.Context(), locale))
+		}
+		w := httptest.NewRecorder()
+		controllers.ForgotPassword(app)(w, req)
+		if w.Code >= 300 {
+			t.Fatalf("forgot-password %s: %d %s", email, w.Code, w.Body.String())
+		}
+	}
+
+	insertUser(t, gdb, "locale-header@example.test", "correct-horse")
+	forgot(t, "locale-header@example.test", "en-GB,en;q=0.9")
+	assertTemplate(t, rec, "golem15.user::mail.restore-en", "locale-header@example.test")
+
+	insertUser(t, gdb, "locale-none@example.test", "correct-horse")
+	forgot(t, "locale-none@example.test", "")
+	assertTemplate(t, rec, "golem15.user::mail.restore", "locale-none@example.test")
+
+	if err := app.Config.Set(mailFallbackKey, "en"); err != nil {
+		t.Fatal(err)
+	}
+	insertUser(t, gdb, "locale-fallback@example.test", "correct-horse")
+	forgot(t, "locale-fallback@example.test", "")
+	assertTemplate(t, rec, "golem15.user::mail.restore-en", "locale-fallback@example.test")
+
+	saved := insertUser(t, gdb, "locale-saved@example.test", "correct-horse")
+	if err := gdb.Model(saved).Update("preferred_locale", "en-GB").Error; err != nil {
+		t.Fatal(err)
+	}
+	forgot(t, "locale-saved@example.test", "pl")
+	assertTemplate(t, rec, "golem15.user::mail.restore-en", "locale-saved@example.test")
+}
diff --git a/mail_subjects_test.go b/mail_subjects_test.go
new file mode 100644
index 0000000..26153bd
--- /dev/null
+++ b/mail_subjects_test.go
@@ -0,0 +1,58 @@
+package user
+
+import (
+	"context"
+	"strings"
+	"testing"
+
+	"git.golem15.com/golem15/summercms/modules/postcard"
+)
+
+// TestPolishMailSubjects renders every Polish template next to its -en twin
+// and requires a subject of its own, so a Polish template with a copied
+// English subject does not slip through.
+func TestPolishMailSubjects(t *testing.T) {
+	p := &Plugin{}
+	cat := postcard.NewCatalog()
+	if err := cat.Register(p.ID(), p.MailTemplatesFS(), p.MailTemplates(), p.MailLayouts()); err != nil {
+		t.Fatal(err)
+	}
+	names := map[string]bool{}
+	for _, name := range p.MailTemplates() {
+		names[name] = true
+	}
+	vars := map[string]any{
+		"name": "Ada", "Name": "Ada",
+		"link": "https://example.com/link", "Link": "https://example.com/link",
+		"code": "1!abc", "Code": "1!abc",
+		"hours": 72, "Hours": 72,
+		"login": "ada@example.test", "Login": "ada@example.test",
+	}
+	subject := func(t *testing.T, template string) string {
+		t.Helper()
+		mem := postcard.NewMemoryDriver()
+		mailer := postcard.NewMailer(cat, mem, postcard.Options{From: "noreply@example.test"})
+		if err := mailer.Send(context.Background(), postcard.Message{Template: template, To: []string{"ada@example.test"}, Vars: vars}); err != nil {
+			t.Fatalf("%s: %v", template, err)
+		}
+		msgs := mem.Messages()
+		if len(msgs) != 1 {
+			t.Fatalf("%s: %d messages", template, len(msgs))
+		}
+		return msgs[0].Subject
+	}
+	pairs := 0
+	for _, name := range p.MailTemplates() {
+		if strings.HasSuffix(name, "-en") || !names[name+"-en"] {
+			continue
+		}
+		pairs++
+		pl, en := subject(t, name), subject(t, name+"-en")
+		if pl == "" || pl == en {
+			t.Errorf("%s: Polish subject %q, English subject %q", name, pl, en)
+		}
+	}
+	if pairs == 0 {
+		t.Fatal("no Polish/English template pairs")
+	}
+}
diff --git a/plugin.go b/plugin.go
index 4d22c51..9d0731d 100644
--- a/plugin.go
+++ b/plugin.go
@@ -76,6 +76,9 @@ func (p *Plugin) Boot(app *backpack.App) error {
 	if err != nil {
 		return err
 	}
+	if _, err := classes.MailFallbackLocale(app); err != nil {
+		return err
+	}
 	// Serve publishes the database after Boot, so the GORM hooks, the
 	// blacklist store and its sweep are installed on lagoon.Publish, or at
 	// once when the database is already published.
diff --git a/views/mail/activate.htm b/views/mail/activate.htm
index 60f399a..518f775 100644
--- a/views/mail/activate.htm
+++ b/views/mail/activate.htm
@@ -1,4 +1,4 @@
-subject = "Activate your account"
+subject = "Aktywuj konto"
 description = "Aktywacja konta"
 layout = "user"
 ==
diff --git a/views/mail/reactivate.htm b/views/mail/reactivate.htm
index 7241977..54f7cfc 100644
--- a/views/mail/reactivate.htm
+++ b/views/mail/reactivate.htm
@@ -1,4 +1,4 @@
-subject = "Welcome back"
+subject = "Witaj ponownie"
 description = "Konto zostało ponownie aktywowane"
 layout = "user"
 ==
diff --git a/views/mail/restore.htm b/views/mail/restore.htm
index de41a80..191e459 100644
--- a/views/mail/restore.htm
+++ b/views/mail/restore.htm
@@ -1,4 +1,4 @@
-subject = "Reset your password"
+subject = "Ustaw nowe hasło"
 description = "Reset hasła"
 layout = "user"
 ==