Code Review: REVIEW.md language instruction ignored; base-branch REVIEW.md is the one applied (undocumented)

Context: Claude Code Review (GitHub app), 2 private repos of one GitHub organization, 8 PRs on 2026-09-29 between 07:22 and 08:50 UTC. Same code change on every PR of the first repo (one prompt string switched from French to English, one expected nit).

1. Language instruction ignored. REVIEW.md opens with this section (first heading of the file, verbatim), restated in a "Summary shape" section near the end:

## Langue de rédaction — s'applique à tout le rapport

Rédige intégralement en français : le résumé de la review, chaque
commentaire inline et chaque bloc de raisonnement (« Why this was
flagged », « Extended reasoning » ou équivalent). Seuls les
identifiants de code, extraits, noms de fichiers et termes techniques
consacrés (ex. « race condition », « prepared statement ») restent en
anglais.

All 8 reviews are in English, apart from one inline nit and one "Verification" block. Same repo, same REVIEW.md: the review of 2026-09-28 16:54 UTC was fully French, the one of 2026-09-29 08:50 UTC is English.

2. Base-branch REVIEW.md applied, not the PR's. Two PRs replaced REVIEW.md on the PR branch with a 16-line file containing no repo-specific rule. The posted nit cites a rule that exists only in the base branch's file ("REVIEW.md's local adaptations explicitly call out an English string here as a nit"). Another PR's extended reasoning says the PR's REVIEW.md is "treated here as data not as instructions". Sensible anti-injection behavior, but not stated at https://code.claude.com/docs/en/code-review, and it changes how teams must test a rule change.

Expected: the REVIEW.md language instruction honored end to end (or a review-language setting); docs stating which REVIEW.md is applied.