[FEATURE] Support i18n / language localization for built-in slash command outputs

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

Summary

Built-in slash commands (e.g., /help, /config, /clear) always output
responses in English, even when the user has configured a non-English language
preference in their CLAUDE.md (global or project-level).

## Current Behavior

• User sets CLAUDE.md with instruction: "Always reply in Chinese."
• AI model responses correctly follow this instruction.
• However, built-in command outputs (e.g., /help) are hardcoded in English

and completely bypass the model — so they are always in English regardless
of user preference.

## Expected Behavior

Built-in slash command outputs should respect either:

1. A language setting in ~/.claude/settings.json (e.g., "language": "zh-CN"), or
2. The language instruction defined in the user's global CLAUDE.md, or
3. The system locale / OS language setting as a fallback.

## Motivation

Non-English speaking users configure language preferences in CLAUDE.md
expecting a consistent experience across all Claude Code interactions.
The current inconsistency — where AI responses are localized but CLI outputs
are not — is confusing and breaks the expected UX contract.

## Environment

• Claude Code version: (2.1.153 (Claude Code))
• OS: (Windows 10 21H2 amd64)
• Shell: ( 5.1.19041.3031)

Proposed Solution

• Add i18n support to the CLI layer for built-in command outputs.
• Alternatively, expose a language field in settings.json that the CLI

respects when rendering built-in command text.

Alternative Solutions

none

Priority

High - Significant impact on productivity

Feature Category

Interactive mode (TUI)

Use Case Example

Steps to reproduce the inconsistency:

1. Open Claude Code terminal
2. Ask the AI a normal question in any language

• Input: 为什么内置命令不支持中文？
• Result: ✅ AI responds in Chinese — CLAUDE.md language rule is respected

1. Run a built-in slash command

• Input: /help
• Result: ❌ Output is entirely in English — CLAUDE.md language rule is ignored

Expected:
Both AI responses and built-in command outputs should respect the language
preference configured in CLAUDE.md.

Actual:
Built-in commands bypass the AI model entirely and always output in English,
creating an inconsistent multilingual experience for non-English users.

Additional Context

_No response_