[BUG] Auto-generated terminal tab title uses wrong language

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?

Claude Code auto-generates a short title for the terminal tab based on the conversation content. In a session conducted entirely in Ukrainian (investigating API endpoints for deleting lessons/courses), the generated tab title
appeared in Chinese instead of matching the conversation language:

研究 API 删除课程端点的行为
```//
(translation: "Research API delete-course-endpoint behavior")

Expected behavior:
The tab title should be generated in the same language as the conversation (Ukrainian in this case), or fall back to English — not switch to an unrelated third language.

Environment:

• Platform: Linux (6.17.9-76061709-generic)
• Shell: /bin/sh
• Conversation language: Ukrainian

Steps to reproduce:

1. Start a Claude Code session and converse entirely in Ukrainian.
2. Let the session run long enough / cover enough content for the harness to auto-generate a tab title.
3. Observe the terminal tab title — renders in Chinese rather than Ukrainian or English.

What Should Happen?

The auto-generated terminal tab title should match the language of the conversation (Ukrainian, in this case), or fall back to English. It should not switch to an unrelated third language (Chinese) that never appeared anywhere in the conversation or its content.

Error Messages/Logs

Steps to Reproduce

1. Start a Claude Code session in a terminal.
2. Conduct the entire conversation in Ukrainian (e.g., ask Claude to investigate backend API endpoints).
3. Let the session progress far enough for the harness to auto-generate/update a short tab title summarizing the conversation.
4. Observe the terminal tab title — it renders in Chinese (研究 API 删除课程端点的行为) instead of Ukrainian or English.

Claude Model

Sonnet (default)

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.206 (Claude Code)

Platform

Anthropic API

Operating System

Other Linux

Terminal/Shell

IntelliJ IDEA terminal

Additional Information

_No response_