Internationalization and Localization Support for CLI Interface

Summary

Add internationalization (i18n) and localization (l10n) support for Claude Code CLI interface messages, command outputs, and help text.

Current Behavior

• All CLI interface messages are in English only
• Command outputs, error messages, and help text are hardcoded in English
• No configuration option to change display language

Requested Behavior

• Support for multiple languages in CLI interface
• Configurable language setting (e.g., via claude config set language ja)
• Localized messages for:
• Command outputs and success/error messages
• Help text and documentation
• Agent creation messages
• Progress indicators and status updates

Use Case

Many non-English speaking developers would benefit from having Claude Code interface in their native language, especially for:

• Better understanding of error messages
• Improved onboarding experience
• Enhanced productivity in local development environments

Proposed Implementation

1. Add language configuration option
2. Implement message localization system
3. Support popular languages (Japanese, Spanish, French, German, etc.)
4. Maintain English as default fallback

Related Issues

This would complement existing internationalization efforts:

• #1547 (Japanese IME input performance issues)
• #2620 (Korean IME composition failure)
• #1553 (IME input rendering lag for non-Latin scripts)
• #1306 (Japanese input UX improvements)
• #678 (Japanese text pasting issues)
• #2780 (Chinese text display corruption)

Together with the existing input-related issues, comprehensive internationalization support would greatly benefit the global developer community using Claude Code.

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Please let me know if a similar issue already exists - I searched through existing issues but may have missed something.