Voice mode: add language configuration (support for non-English languages)

Feature Request

Problem

The /voice mode currently has no language configuration option. It appears to be English-only or relies on automatic language detection, which doesn't work reliably for non-English languages like Portuguese (pt-BR).

Proposed Solution

Add a configurable language setting for voice mode, either via:

• A voiceLanguage field in settings.json (e.g., "voiceLanguage": "pt-BR")
• A CLI flag like --voice-language pt-BR
• An environment variable like CLAUDE_VOICE_LANGUAGE=pt-BR

Use Case

Many Claude Code users are non-English speakers who would benefit from using voice mode in their native language. For example, Brazilian Portuguese speakers working on legal tech products need to dictate commands and descriptions in Portuguese.

Current Workaround

Users must type in Portuguese instead of using voice mode, or install third-party MCP servers for speech-to-text with language support.

Additional Context

The underlying Whisper model already supports 50+ languages including Portuguese — this is primarily a configuration exposure issue, not a capability limitation.