Add language configuration for /voice command

Feature Request

Summary

The /voice command currently has the language hardcoded to English. When speaking in Japanese (or other non-English languages), the speech is incorrectly transcribed as English text.

Problem

• The Whisper API language parameter appears to be fixed to English
• There is no user-facing setting to change the voice recognition language
• Non-English speakers cannot use /voice effectively, as their speech is converted to English regardless of the spoken language

Proposed Solution

Add a configurable language setting for /voice, for example:

• A setting in ~/.claude/settings.json (e.g., "voice.language": "ja")
• Or a flag like /voice --lang ja
• Or auto-detect the language from Whisper API (by not specifying the language parameter)

Environment

• Claude Code version: 2.1.72
• OS: macOS (Darwin 25.3.0, Apple Silicon)