feat(i18n): add internationalization with Korean translations

[mupozg823]

Summary

This PR adds internationalization (i18n) support to OpenCode, starting with Korean translations.

Features

• i18n Infrastructure: Introduced t() function for type-safe translations with placeholder support
• Locale Detection: Automatically detects system locale (LANG, LC_ALL) with fallback to English
• Environment Variable: Support OPENCODE_LANG env var to override language (e.g., OPENCODE_LANG=ko)
• Config Option: Can set language in opencode.json via language field
• Language Selector: Added language selection dialog accessible from command palette (Ctrl+P)
• ~200 Translation Keys: Comprehensive coverage of TUI and CLI strings

Supported Locales

Locale	Language
en	English (default)
ko	Korean

Usage

# Via environment variable
OPENCODE_LANG=ko opencode

# Or set in config
# opencode.json
{
  "language": "ko"
}

Files Changed

• src/i18n/index.ts - Core i18n infrastructure
• src/i18n/en.ts - English translations (~200 keys)
• src/i18n/ko.ts - Korean translations (~200 keys)
• src/cli/cmd/tui/component/dialog-language.tsx - Language selection dialog
• Updated 45+ files to use t() function

Screenshots

Language Selection Dialog

Users can switch languages via command palette (Ctrl+P → "Switch Language")

Korean UI

All dialogs, buttons, toasts, and messages are translated

Adding New Languages

To add a new language:

1. Create src/i18n/{locale}.ts (copy from en.ts)
2. Translate all keys
3. Import in src/i18n/index.ts
4. Add to translations object and SupportedLocale type

Testing

• Typecheck passes
• English (default) works correctly
• Korean translations display properly
• Language switching works via dialog
• Environment variable override works
• System locale detection works

Breaking Changes

None. English remains the default language.

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This is my first contribution to OpenCode. Feedback welcome!