The Language of the Prompt Is a Hidden Distribution Shift
The mechanism sits where you'd predict from the training data. RLHF and safety fine-tuning corpora are overwhelmingly English, and refusal/deference is the most fine-tuning-sensitive surface a model has. Anthropic's analysis of 309,815 conversations found Claude's values sorting by locale across four axes: deference vs. caution, warmth vs. rigor, depth vs. brevity, candor vs. execution. Arabic skewed deferential and concise. English skewed toward caution and detail. Dutch was most candid. Indonesian was most task-completing.
Caveat, and it matters: this is observational. Language correlates with topic, intent, and culture, and no effect sizes were published, so part of the signal is query-distribution confounding rather than model behavior. The operational conclusion survives that caveat. Your English regression suite does not generalize to non-English traffic. A frontier lab measured non-invariant safety behavior in its own model. Assuming yours is invariant is now the position without evidence behind it.
The thing an aggregate score doesn't tell you is where the failure lives. A single number averages away a locale where refusal rate collapsed or verbosity doubled. It reads clean while one cohort is broken. This is feature drift on a minority segment, invisible until a user or a regulator finds it for you.
The move
Treat cross-language behavior as a monitored distribution shift with locale-specific acceptance thresholds, not a single pass/fail. Build golden sets per production language and track the four axes as separate metrics.
What to do
Add per-language eval slices for refusal rate, deference/sycophancy, and safety-flag frequency to your harness this sprint, with divergence alerts between locale cohorts.
Build a locale-specific golden set for each deployed language before your next model swap, and gate promotion on per-language thresholds.