[FEATURE] Add interface localization (i18n) — translate the CLI UI to other languages (e.g. Spanish)

Preflight Checklist

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

Problem Statement

Claude Code can reply in my configured language (Spanish), but the entire CLI
interface remains English-only: menus, /config, /help, permission prompts,
status line, system messages, and error text. As a non-native English speaker,
this slows me down and adds friction when onboarding teammates who don't read
English comfortably. There is currently no setting or locale file to change the
interface language.

Proposed Solution

Add an interface language setting, e.g. a language / locale option in the
settings file (or a /config entry), that localizes the CLI UI strings. Ideally:

• Respect the OS locale by default, with a manual override.
• Ship community-translatable string catalogs (e.g. JSON/po files) so the

community can contribute translations.

• Start with a few high-demand languages (Spanish, Portuguese, French, etc.).

Model responses would still follow the existing response-language config; this
request is only about the UI chrome.

Alternative Solutions

• Setting "respond in Spanish" via CLAUDE.md — only affects the model's replies,

not the interface.

• Reading the English UI directly — works but is a barrier for non-English users

and for onboarding Spanish-speaking teammates.
No real workaround exists for the interface itself.

Priority

High - Significant impact on productivity

Feature Category

CLI commands and flags

Use Case Example

1. I install Claude Code and set my language to Spanish.
2. I run /config and the menu labels appear in Spanish.
3. A tool wants to run a command; the permission prompt ("Allow / Deny",

explanations) shows in Spanish so I understand exactly what I'm approving.

1. I onboard a Spanish-speaking teammate who can navigate /help and the status

line without needing English.

Additional Context

Many dev CLIs and IDEs (VS Code, JetBrains, gcloud) offer UI localization.
Even partial coverage of the most frequent strings (permission prompts, /help,
/config, error messages) would meaningfully lower the barrier for non-English
speakers.