Proposal: make the app UI localizable, starting with Traditional and Simplified Chinese

[yatping]

Proposal

Make FluidVoice's UI localizable, then add Traditional Chinese (zh-Hant) and Simplified Chinese (zh-Hans) as the first translations.

Why

FluidVoice dictates in many languages, but the app itself is English-only: knownRegions is en, Base, there is no String Catalog, and none of the roughly 1,300 SwiftUI string literals in Sources/ go through localization. People who dictate in Chinese, Japanese, Korean and other languages still have to set up and troubleshoot the app in English.

Scope I'd suggest (one PR per step, each with screenshots)

1. Infrastructure only, no visible change. Add Localizable.xcstrings (and an InfoPlist.xcstrings for the permission prompts), and add the locales to knownRegions. SwiftUI Text("…"), Button("…"), Toggle("…") and similar already take LocalizedStringKey, so most call sites need no code change. Only String values built in view models move to String(localized:). English output stays byte-for-byte the same.
2. zh-Hant + zh-Hans translations, keeping product, model and provider names, key symbols and format specifiers unchanged.
3. Optionally, a short docs/localization.md so other contributors can add languages without touching code.

I'd keep each PR focused and separate from feature work.

Questions for maintainers

• Is localization something you'd accept, and is this split the right shape?
• Any area you'd rather keep English-only for now (for example, debug or developer settings)?

Once you approve, I'll open a tracking issue and link it here as CONTRIBUTING asks.