UI Localization / i18n support — Portuguese (pt-BR) and other languages (reopen of #66637)

This is a re-submission of #66637, which was auto-closed by the stale bot for inactivity ("Closing for now — inactive for too long"), not because the request was rejected on its merits. The underlying need is still valid and relevant.

Problem Statement

The Claude Code interface is only available in English. All menus, commands, status messages, tooltips, and UI text are displayed in English regardless of the user's language preference or system locale.

This creates a barrier for non-English speakers — particularly the large Portuguese-speaking developer community (Brazil + Portugal) — who must navigate an English-only interface even when they have "language": "portuguese" configured in settings.json (which only affects Claude's responses, not the UI itself).

Proposed Solution

Add internationalization (i18n) support to Claude Code so that the UI language can be configured separately from the AI response language:

• Add a "uiLanguage" setting to settings.json (e.g., "uiLanguage": "pt-BR")
• Or extend the existing "language" setting to also control UI language
• Translate interface text: menus, slash command descriptions, status messages, error messages, spinner verbs, tips, and prompts

Initial language targets: Portuguese (pt-BR), Spanish (es), French (fr), German (de), Japanese (ja), Chinese (zh-CN)

Related

• Original request: #66637 (closed by stale bot, not by maintainer decision)
• Consolidated proposal covering CLI i18n more broadly: #70490