[FEATURE] Add internationalization (i18n) support for global users

[hoteye]

Preflight Checklist

• I have searched existing requests and this feature hasn't been requested yet
• This is a single feature request (not multiple features)

Problem Statement

Claude Code currently only supports English interface, creating barriers for non-English speaking developers worldwide. When developers use Claude Code in their native language environment, they encounter:

• All command descriptions and help text in English only
• Error messages that are harder to understand and debug
• Settings interfaces that require English comprehension
• Reduced productivity due to language barriers

This limits Claude Code's accessibility for the global developer community, particularly in markets like Japan, Latin America, and non-English speaking Europe where developers would be more productive with native language support.

The current workflow requires developers to constantly translate English terms mentally, lookup unfamiliar technical vocabulary, and interpret error messages in a second language, which slows down development and increases cognitive load.

Proposed Solution

I'd like Claude Code to automatically detect the system locale (LANG, LC_ALL environment variables) and display the interface in the user's preferred language. I've implemented comprehensive i18n support in a similar CLI tool. Please see my PR at google-gemini/gemini-cli#6832.

The interface should show:

• Translated command descriptions and help text
• Localized error messages and diagnostics
• Multi-language settings configuration via /config command
• Support for major languages: English, Japanese, Spanish, French, German

There should be multiple configuration methods:

# Via standard UNIX environment variables
export LANG=ja_JP.UTF-8

# Via CLI flag
claude-code --lang ja

# Via interactive config command
/config
> Language: [English] → 日本語 → Español

# Via configuration file
{
  "language": "ja"
}

Alternative Solutions

Priority

Priority

High - Significant impact on productivity

Feature Category

Interactive mode (TUI)

Use Case Example

Scenario: A Japanese developer working on a React project encounters an authentication error.

Current Experience:

$ claude-code
Error: Authentication required. Please run 'claude-code auth' first.
Available commands: help, auth, settings, quit

Developer has to translate error message and commands manually

With i18n Support:

$ LANG=ja_JP.UTF-8 claude-code
エラー：認証が必要です。まず 'claude-code auth' を実行してください。
利用可能なコマンド：help, auth, settings, quit

Step-by-step workflow:

1. Developer sets export LANG=ja_JP.UTF-8 in shell profile
2. Launches Claude Code - interface automatically appears in Japanese
3. Gets clear error message in native language with precise instructions
4. Uses /config command to permanently save language preference
5. All future sessions use Japanese interface by default
6. Developer is more productive with reduced cognitive load

Additional Context

Reference Implementation:

• 4 languages supported with 98%+ translation coverage
• Minimal performance impact (~20KB bundle size increase)
• Type-safe translation keys with namespace organization
• Runtime language switching capability

Technical Approach:

• Use i18next framework for lightweight, proven i18n support
• Modular translation files organized by feature (commands, errors, help)
• Automatic locale detection with manual override options
• Seamless integration with existing CLI architecture

Global Impact: This feature would significantly lower barriers for international developers and expand Claude Code's market reach in non-English speaking regions.

[github-actions]

Found 2 possible duplicate issues:

1. Internationalization and Localization Support for CLI Interface #4866
2. [Request]  #2415

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[github-actions]

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