Language instruction ignored: Claude responds in Japanese despite explicit Traditional Chinese requirement in CLAUDE.md

Summary

Claude Code repeatedly responds in Japanese or Korean despite an explicit instruction in CLAUDE.md to always use Traditional Chinese (台灣正體中文) and never use Japanese, Korean, or Simplified Chinese.

Environment

• Claude Code version: unknown (VSCode extension)
• Model: claude-sonnet-4-6
• Platform: macOS (darwin 24.6.0)

CLAUDE.md instruction

## 語言設定

永遠使用台灣正體中文回覆，不得使用日文、韓文或簡體中文。

What happened

This has occurred across multiple sessions. Claude switches to Japanese or Korean mid-conversation during technical work, despite the CLAUDE.md instruction being loaded and visible in context.

Example 1 (screenshot provided)

After a git push operation, Claude responded entirely in Japanese — then caught itself, apologized in Japanese, and continued to respond in Japanese while attempting to correct the issue:

commit と push 完了。lint-staged の index.lock エラーは先ほどの background プロセスの残骸で、最終的には成功している。 申し訳ありません、日本語になっていました。再度、正體中文で： storefront に push 完了しました。...

The "correction" was itself still in Japanese.

Example 2 (this session)

After a gh pr view tool call returning English-only JSON, Claude responded in Japanese:

Backend tests が FAILURE のままです。CI が通っていない状態では merge できません。 ただ、このPR（chore/codex-pr-review-agents）の内容は AGENTS.md への変更で...

Example 3 (reported, no screenshot)

The user has also received Korean responses on separate occasions.

What should have happened

All responses should be in Traditional Chinese regardless of the operation being performed.

Key observations

• The project codebase contains only Traditional Chinese and English — no Japanese or Korean anywhere in the repo
• All team members write in Traditional Chinese; other team members do NOT experience this issue with their own Claude Code sessions
• The violation occurs after tool calls (git, gh) that return English-only output — there is no Japanese or Korean content in the conversation context that could explain the language switch
• After being corrected, Claude sometimes apologizes and "switches back" — but the apology itself is still in Japanese
• The issue recurs across sessions despite being recorded in both CLAUDE.md and the auto-memory system
• The pattern is specific to this one user, suggesting something in their session context or environment triggers it — but what that is remains unknown