Add i18n support / Portuguese (pt-BR) localization for built-in slash commands

Problem

Built-in slash command descriptions are hardcoded in English inside the Claude Code binary. Affected commands include:

• /verify
• /code-review
• /loop
• /schedule
• /claude-api
• /run
• /init
• /review
• /security-review
• /update-config
• /keybindings-help
• /fewer-permission-prompts

User-defined commands in ~/.claude/commands/ can be written in any language, but built-ins can't be overridden without patching the Mach-O binary (fragile, breaks on every update).

Request

1. Add an i18n layer so UI strings and slash-command descriptions can be localized.
2. Ship pt-BR (Brazilian Portuguese) as one of the supported locales.
3. Allow selection via settings.json or an env var (e.g. CLAUDE_LANG=pt-BR).

Why it matters

• Improves adoption in LATAM/Brazil, where Claude Code usage is growing.
• Users already interact with Claude in their native language; having the CLI chrome match is a natural next step.
• Other dev tools (VS Code, GitHub CLI, JetBrains) ship localized UI as standard.

Environment

• Claude Code version: 2.1.147
• Platform: macOS (Darwin 25.5.0, arm64)