Support for configurable CLI language / i18n (zh-CN, ja, etc.)

Problem

Claude Code's CLI interface is entirely English-only. All prompts, menus, help text, status messages, and error messages are hardcoded in English with no localization path. This creates friction for the large population of Chinese-speaking developers and developers in other non-English-speaking communities.

Previous requests (#4866, #22356) were closed without resolution.

Proposed solution

1. Externalize all user-facing CLI strings into locale resource files
2. Add a config option: claude config set language zh-CN (or ja, es, fr, de, ko, etc.)
3. Ship with an initial zh-CN locale covering:

• Permission prompts
• Progress indicators and status messages
• Help text and command descriptions
• Error messages

1. Allow community-contributed translations

Scope

• CLI strings only (not the AI model's output language, which is already configurable via CLAUDE.md / system prompt)
• Language setting is user-level, not project-level

Why this matters

The AI assistant already handles Chinese conversations perfectly — the last remaining piece is the surrounding tool UI. A localized CLI completes the experience and lowers the barrier for millions of developers.

# Example of what it should look like
$ claude config set language zh-CN
语言已设置为: 简体中文 (zh-CN)

$ claude --help
用法: claude [选项] [提示词]
...

Happy to contribute Chinese translations if the team opens up the strings.