- preferred_locale and the request locale are normalised (comma lists,
q-weights, region subtags, case) to pl or en by NormalizeMailLocale, so a
raw Accept-Language such as en-GB,en;q=0.9 selects the -en templates
- new golem15.user.mail.fallback_locale (default pl) is used when neither
names a supported language; ResolveMailLocaleFor applies it, and Boot
rejects any value other than pl or en
- activate, reactivate and restore Polish templates get Polish subjects
- no template, route or payload changes; MailTemplate and
ResolveMailLocale keep their signatures
