- Add separate translation_language setting (independent from interface language)
- Implement 3-step onboarding for new users:
1. Choose interface language
2. Choose learning language
3. Choose translation language
- Fix localization issues when using callback.message (user_id from state)
- Add UserService.get_user_by_id() method
- Add get_user_translation_lang() helper in i18n
- Update all handlers to use correct translation language
- Add localization keys for onboarding (ru/en/ja)
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
