i18n: Support localized slash command descriptions

Feature Request

Problem

Claude Code's slash command descriptions (shown in the autocomplete menu when typing /) are hardcoded in English. For non-English speaking users, this creates a friction point in the UX.

Proposal

Add internationalization (i18n) support for:

1. Built-in slash command descriptions (/help, /clear, /cost, /compact, /review, /init, etc.) — translate the short descriptions shown in the autocomplete dropdown
2. System messages and UI text — status messages, prompts, and other user-facing strings
3. Locale detection — auto-detect from system locale ($LANG, $LC_ALL) or allow explicit config via settings.json:

``json
{
"language": "es"
}
``

Priority languages

Spanish, Portuguese, French, German, Japanese, Chinese, Korean — covering the majority of non-English developer populations.

Context

Claude Code is increasingly used by teams worldwide. The AI responses are already multilingual (Claude adapts to the user's language), but the CLI chrome (slash command hints, status text) remains English-only. Localizing these strings would make the tool feel native for international users.

Implementation notes

• Slash command descriptions are short strings — a simple JSON locale file per language would suffice
• User-created skills already support custom descriptions, so the infrastructure for display is there
• This could be community-contributed (translation PRs) once the i18n framework is in place

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