Feature Request: Interface localization / i18n support

Summary

Add support for localizing the Claude Code CLI interface (menus, system messages, prompts, etc.) into different languages.

Motivation

Currently, only the AI responses can be configured to use a specific language (via CLAUDE.md or prompt instructions). However, the entire CLI interface — including system messages, prompts, and menus — remains in English with no option to change it.

For non-English speaking users, having the full interface available in their native language would greatly improve the user experience and accessibility.

Suggested languages (initial priority)

• pt-br (Portuguese - Brazil)
• es (Spanish)
• fr (French)
• de (German)
• ja (Japanese)
• zh (Chinese)

Proposed solution

• Add an i18n/localization layer to the CLI
• Allow users to set their preferred language via settings (e.g., claude config set language pt-br)
• Fall back to English if a translation is not available