[BUG] Session summary generated in Russian for a Czech-language conversation (2.1.222, Linux)

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?

Related to #64408 (open), #68628, #68532, #45712 — filing separately because this
instance affects the session summarizer rather than the main conversation output.

Claude Code generated the session summary in Russian for a conversation conducted
entirely in Czech. Other sessions in the same project on the same day summarized
correctly in English.

The wrong-language summary appears in two places: the /resume session picker and
the OSC 0 terminal title. Since the picker reads from the session store rather than
from the terminal, the summary originates in the summarizer itself, not in terminal
rendering or in any local process writing escape sequences to the TTY.

The summary is semantically correct — it accurately describes the task being worked
on ("Improve redirect button behavior and chat display"), matching the branch name
and the modified files. Only the language is wrong.

Environment verified clean before filing:

• ~/.claude/settings.json contains only {"model": "opus[1m]"}
• No ANTHROPIC_BASE_URL, no ANTHROPIC_API_KEY, no HTTPS_PROXY, no TLS overrides
• No such variables in shell profiles or in /proc/<pid>/environ of running sessions
• Direct connection to api.anthropic.com
• All MCP servers are remote HTTP on official domains; no stdio servers

What Should Happen?

The session summary should be generated in the language of the conversation
(Czech), or consistently in English. It should not be generated in a third
language that appears nowhere in the conversation, the project, or the
configuration.

Error Messages/Logs

No error or stack trace — this is a generation-quality issue, not a crash.

The artifact is the summary string itself, as shown in the /resume picker:

  Улучшить поведение redirect button и display чата
  2 hours ago · bel-dmar-bart-tour-mode-init-by-app-page-redir · 911.7KB

The next session in the same project, same day, summarized normally:

  Audit AI chat data accuracy and email sender investigation
  5 hours ago · bel-dmar-bart-fix-tools-data-correctness · 6.6MB

Steps to Reproduce

Not reliably reproducible — appears non-deterministic, consistent with the other
reports linked above.

Conditions observed when it occurred:

1. Long agentic session in a Laravel/Vue project, heavy Bash tool use
2. All user prompts in Czech, technical identifiers in English (normal code-switching

for a Czech developer)

1. Session transcript ~912 KB
2. The summary was generated in Russian; no Russian appears anywhere in the prompts,

the codebase, the branch names, or the configuration

1. Other sessions in the same project on the same day summarized correctly in English

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.222 (Claude Code)

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

Other

Additional Information

Terminator / bash

<img width="509" height="161" alt="Image" src="https://github.com/user-attachments/assets/538e9594-d80c-462e-97b3-bfb07c528f88" />

<img width="513" height="38" alt="Image" src="https://github.com/user-attachments/assets/d2f5de7a-f801-4de6-b098-0d6a935dc0c8" />

<img width="390" height="31" alt="Image" src="https://github.com/user-attachments/assets/eb2035e4-52f9-4043-898a-32460d803243" />