Opus 5.5 / Sonnet 5.5 drift into English mid-session despite German user input

Observed
In Claude Code sessions with Opus 5.5 and Sonnet 5.5, the model repeatedly switches its replies to English during a session, even though all user messages are in German. Once it has switched, it keeps answering in English for the rest of the session instead of returning to German.

Expected
Replies stay in the language of the user's messages (German) throughout the session, as with earlier model generations.

Impact
The user is fluent in English, but has to keep switching mental context and re-asking for German. It happens in long, tool-heavy sessions.

Environment

• Claude desktop app (Code tab), Windows 11
• Models: Opus 5.5, Sonnet 5.5
• Global CLAUDE.md is written in German, with no explicit language rule