[BUG] Claude spontaneously switches to Korean mid-conversation when using Chinese

Environment

• Claude Code CLI Version: 2.1.63
• Model: claude-sonnet-4-6
• OS: macOS Darwin 24.6.0
• Conversation language: Simplified Chinese

Description

During Chinese-language conversations, Claude occasionally produces entire responses in Korean with no prompt, warning, or apparent trigger. This has happened multiple times across different sessions, always defaulting to Korean specifically — not Japanese, English, or other languages.

Steps to Reproduce

No reliable repro steps identified yet. The switch appears to happen spontaneously, often after:

• Spawning a sub-agent (Agent tool call)
• Receiving tool results that contain mixed-language content

Expected Behavior

Claude should maintain the conversation language (Simplified Chinese) regardless of what language tool results or sub-agent outputs are in.

Actual Behavior

Claude produces a full response in Korean. Example from session:

솔직히 말씀드리면 — 이전 세션의 대화 내용에 접근할 수 없습니다. 저는 이번 대화만 기억하고, 오늘 아침 세션은 독립된 컨텍스트였습니다.

Additional Notes

• Reproducible across multiple sessions (not a one-time fluke)
• Korean is consistently the fallback language, never Japanese or others
• Possibly triggered when a sub-agent returns content in a non-Chinese language
• Related: #21446 (German conversation regression), but distinct — this is a full language switch, not vocabulary leakage

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