Voice mode: add multi-language support (currently English-only)

Description

Voice mode (/voice) currently only transcribes in English. There is no configuration option to set the transcription language (e.g., voiceLanguage in settings.json).

Use case

Non-English speaking users (e.g., Spanish) cannot use voice mode effectively. The transcription engine ignores the system language settings (Windows Speech Language) and always outputs English.

Proposed solution

Add a voiceLanguage setting in settings.json (or auto-detect from system locale) so users can configure the speech-to-text language. Example:

{
  "voiceLanguage": "es"
}

Alternatively, if using Whisper or a similar model that supports auto-detection, ensure the language detection actually works for non-English input.

Environment

• Platform: Windows 11 Pro
• Claude Code version: latest
• System language: Spanish