[kevmo314]

Spoken TTS mode/backlog announcements and the chat author prelude currently use the UI's language even when chat is read in another language. An English UI therefore announces English system text during a German or Japanese stream.

Choose the announcement translation from the current reading language, including primary-stream language and regional variants. Mode on/off, alerts-only, and backlog shutdown use that translation and its matching neural/system voice. Use all generated app locales, including the expanded UI translations in #1736. Where no app translation exists, retain the UI translation and match its voice to the text. The chat author prelude (including “said”) uses each message’s channel/TTS language, preserving the viewer name and original message text. Other event-alert strings retain their existing UI-language policy; arbitrary system text remains verbatim. System-engine alert speech now receives its text's locale explicitly instead of relying on the device default.

Validation: the Flutter suite and announcement regressions pass, including every generated app locale, translated mode announcements, primary-stream selection, regional tags, untranslated fallback, backlog shutdown, and existing voice/routing behavior. Targeted Dart analysis and diff checks pass.

Closes #1729.