[BUG] Language instruction in CLAUDE.md ignored — responses randomly switch to Japanese/Korean

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Despite CLAUDE.md specifying "永遠用繁體中文回覆" (Always respond in Traditional Chinese), responses randomly switch to Japanese or Korean mid-session without any trigger.

What Should Happen?

All responses should be in Traditional Chinese throughout the entire session, as instructed in CLAUDE.md.

Error Messages/Logs

Steps to Reproduce

No consistent reproduction steps found. Confirmed conditions when it occurred:

• No Japanese or Korean content was pasted in the conversation
• No subagents were invoked
• No context compaction occurred
• CLAUDE.md was not modified recently
• Happened across multiple separate sessions over 2 days (Japanese), and again today (Korean)

Claude Model

None

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

2.1.144（Claude Code）

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_