End-to-end i18n and localization support

[justin13888]

Description

All native clients and server have to cooperate...

Use Case

Our users are international.

Proposed Solution (optional)

TODO

[justin13888]

We will crowdsource translations but keep format portable for LLM-based backfilling.

Here is the tentative list of locales:

[
  { "locale": "en-US", "name": "English (United States)", "fallbacks": ["en-GB", "en-AU", "en-CA", "en-IN", "en"] },
  { "locale": "zh-Hans", "name": "Chinese (Simplified)", "fallbacks": ["zh-CN", "zh-SG", "zh"] },
  { "locale": "zh-TW", "name": "Chinese (Traditional - Taiwan)", "fallbacks": ["zh-Hant", "zh-MO"] },
  { "locale": "zh-HK", "name": "Chinese (Traditional - Hong Kong)", "fallbacks": ["zh-TW", "zh-Hant"] },
  { "locale": "hi-IN", "name": "Hindi (India)", "fallbacks": ["hi"] },
  { "locale": "es-419", "name": "Spanish (Latin America)", "fallbacks": ["es-MX", "es-ES", "es-AR", "es"] },
  { "locale": "ar-SA", "name": "Arabic (Modern Standard)", "fallbacks": ["ar-EG", "ar-AE", "ar"], "dir": "rtl" },
  { "locale": "bn-BD", "name": "Bengali (Bangladesh)", "fallbacks": ["bn-IN", "bn"] },
  { "locale": "pt-BR", "name": "Portuguese (Brazil)", "fallbacks": ["pt-PT", "pt"] },
  { "locale": "ru-RU", "name": "Russian (Russia)", "fallbacks": ["ru"] },
  { "locale": "id-ID", "name": "Indonesian", "fallbacks": ["id"] },
  { "locale": "ur-PK", "name": "Urdu (Pakistan)", "fallbacks": ["ur-IN", "ur"], "dir": "rtl" },
  { "locale": "ja-JP", "name": "Japanese (Japan)", "fallbacks": ["ja"] },
  { "locale": "fr-FR", "name": "French (France)", "fallbacks": ["fr-CA", "fr-CH", "fr-BE", "fr"] },
  { "locale": "de-DE", "name": "German (Germany)", "fallbacks": ["de-AT", "de-CH", "de"] },
  { "locale": "te-IN", "name": "Telugu (India)", "fallbacks": ["te"] },
  { "locale": "mr-IN", "name": "Marathi (India)", "fallbacks": ["mr"] },
  { "locale": "ta-IN", "name": "Tamil (India)", "fallbacks": ["ta-LK", "ta-SG", "ta"] },
  { "locale": "vi-VN", "name": "Vietnamese", "fallbacks": ["vi"] },
  { "locale": "tr-TR", "name": "Turkish", "fallbacks": ["tr"] },
  { "locale": "ko-KR", "name": "Korean (South Korea)", "fallbacks": ["ko"] },
  { "locale": "fa-IR", "name": "Persian (Iran)", "fallbacks": ["fa-AF", "fa"], "dir": "rtl" },
  { "locale": "gu-IN", "name": "Gujarati (India)", "fallbacks": ["gu"] },
  { "locale": "pa-Guru-IN", "name": "Punjabi (Gurmukhi, India)", "fallbacks": ["pa-IN", "pa"] },
  { "locale": "ha-NG", "name": "Hausa (Nigeria)", "fallbacks": ["ha"] },
  { "locale": "sw-KE", "name": "Swahili (Kenya)", "fallbacks": ["sw-TZ", "sw"] },
  { "locale": "kn-IN", "name": "Kannada (India)", "fallbacks": ["kn"] },
  { "locale": "th-TH", "name": "Thai", "fallbacks": ["th"] },
  { "locale": "fil-PH", "name": "Filipino", "fallbacks": ["tl", "fil"] },
  { "locale": "am-ET", "name": "Amharic (Ethiopia)", "fallbacks": ["am"] },
  { "locale": "ml-IN", "name": "Malayalam (India)", "fallbacks": ["ml"] },
  { "locale": "my-MM", "name": "Burmese", "fallbacks": ["my"] },
  { "locale": "pl-PL", "name": "Polish", "fallbacks": ["pl"] },
  { "locale": "uk-UA", "name": "Ukrainian", "fallbacks": ["uk"] },
  { "locale": "or-IN", "name": "Odia (India)", "fallbacks": ["or"] }
]

Ensure our locale format holds both the key to translation mapping, the history (who contributed that which could be a human or LLM, and a timestamp). Format should support variable substitutions but also syntactically verify every key in each translation has all the variables expected.

Consider which collaboration platform is most suitable (e.g., Weblate).

[justin13888]

Define new github issue templates for:

1. Report translation error/missing translations. Must provide fix in at least their own native language via our platform as an example. (Capsule contributors will backfill any issues surfaced from report.)
2. New language request

[justin13888]

Design is now pre-planned; this issue is closed by implementation of Lane I in SLICES.md.

• The i18n infrastructure already ships (ICU catalogs under locales/, xtask i18n codegen to Rust/web/Android/iOS, capsule-i18n runtime, the error.* code contract) — see the i18n design doc.
• The official language set is now committed in i18n §Supported Languages: en + zh-Hans, zh-Hant, ja, ko, fr, de, es, pt-BR, it, ru, hi, ar (RTL in scope via Arabic; zh-Hant falls back to en, never zh-Hans). locales/config.json remains the mechanical truth of what ships.
• Slices: S-I1 migrates the remaining hardcoded strings (web JSX, SwiftUI, Compose) onto catalog keys with a regression gate; S-I2 rolls out the twelve catalogs + RTL support.