chore(locale): document formatting as a non-goal, ignore the locale cache dir

The release ships no per-locale CLDR data (negotiation needs none) and runtime download is off, so any future Localize formatting call would raise LocaleNotFoundInCacheError for non-English locales. State that next to the config, and ignore /priv/localize/ so a manual `mix localize.download_locales` cannot leave committable blobs in priv.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: JakobLichterfeld

.gitignore

@@ -34,6 +34,10 @@ npm-debug.log
 # Ignore digested assets cache.
 /priv/static/cache_manifest.json
 
+# CLDR locale data cache (mix localize.download_locales); the release
+# deliberately ships none — Localize is used for negotiation only.
+/priv/localize/
+
 # Files matching config/*.secret.exs pattern contain sensitive
 # data and you should not commit them into version control.
 #

config/config.exs

@@ -30,6 +30,13 @@ config :phoenix,
 
 config :gettext, :default_locale, "en"
 
+# Localize is used for UI-language negotiation only (translations come
+# from Gettext, dates from Timex). The release deliberately ships no
+# per-locale CLDR data — negotiation needs none — so Localize formatting
+# APIs (Number/DateTime/Unit) would raise LocaleNotFoundInCacheError for
+# any locale but "en". Keep formatting a non-goal, or wire
+# `mix localize.download_locales` into the build first.
+#
 # Static list so compile-time Localize calls see the same set as runtime.
 # Use CLDR IDs (hyphens, :"zh-Hans") — expanding Gettext names like
 # "zh_Hans" collapses to :zh via likely subtags and loses zh-Hant.