[BUG] Model mixes up East Asian languages

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?

Language setting and system prompt both set to Chinese, but model repeatedly outputs Korean

Environment:

• Claude Code CLI
• Model: claude-sonnet-4-6[1m]
• language: "Simplified Chinese" set in ~/.claude/settings.json
• System prompt explicitly states: "Always respond in Simplified Chinese"

Bug:
Despite both the language setting and the system prompt requiring Simplified Chinese, the model repeatedly produces responses in Korean instead. This happens
unpredictably — sometimes within the same conversation it switches between Chinese, Korean, and English with no trigger from the user.

Impact:
The language setting is effectively non-functional for Chinese users. Every session requires manual policing of the output language. This is a critical UX
failure for non-English locales.

What Should Happen?

It should keep outputing in Chinese, but it keeps outputting Korean. It realizes it but cannot act correctly. Happens every day every session

Error Messages/Logs

Steps to Reproduce

Steps to reproduce:

1. Set "language": "Simplified Chinese" in ~/.claude/settings.json
2. Add system instruction: "Always respond in Simplified Chinese"
3. Have a multi-turn conversation
4. Observe model randomly outputting Korean

Expected behavior:
All responses in Simplified Chinese, every turn, without exception.

Actual behavior:
Model intermittently switches to Korean. User must repeatedly correct it. Even after correction, it reverts to Korean in subsequent turns.

Claude Model

Opus

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

2.1.116

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

<img width="1133" height="706" alt="Image" src="https://github.com/user-attachments/assets/5692d2ad-15c9-4c95-b399-dc0aa2571de7" />