[Bug] German language regression — English words leak into German responses + grammar errors (2026-01-28)

Environment

• Claude Code CLI Version: 2.1.22
• Model: claude-sonnet-4-5-20250929 (Sonnet 4.5)
• OS: Darwin 25.2.0 (macOS)
• Date: 2026-01-28
• Language setting: German (Deutsch) as primary response language

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Description

Two distinct failure modes observed in German-language conversations today:

1. English words leaking into German sentences — no technical justification, pure German equivalents exist
2. Grammar/declension errors — wrong adjective endings for neuter nouns

Both issues appear to have started today and were not observed in prior sessions.

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Examples

From another Claude Code session (reported by user):

| Output | Expected | Error Type |
|--------|----------|------------|
| bessere Startlayout | besseres Startlayout | Neuter declension (-e → -es) |
| Laut mir kurz in den aktuellen Code | Schauen wir kurz in den aktuellen Code | Completely nonsensical sentence structure |

From this session (self-observed by Claude):

| Output | Expected | Error Type |
|--------|----------|------------|
| komplett garbled | komplett verstümmelt | English word in German sentence |
| kein Reason | kein Grund | English word in German sentence |

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Impact

• Undermines trust in German-language outputs
• Affects professional communication quality
• Pattern suggests model is under implicit pressure to abbreviate, causing language discipline to degrade

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Steps to Reproduce

1. Open Claude Code CLI
2. Communicate entirely in German
3. Request code analysis or technical tasks
4. Observe: English terms and grammar errors appear in responses

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Expected Behavior

All output in the declared language (German), with correct grammar, declension, and zero unjustified English loanwords.