diff --git a/CLAUDE.md b/CLAUDE.md
index f22f44af..ceec0656 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -29,6 +29,10 @@ pnpm format-and-lint # Lint and format
 - Use `zod` for runtime validation schemas
 - Prefer explicit imports over barrel re-exports
 
+## Language
+
+- All artifacts must be written in English: code, comments, commit messages, issues, PRs, documentation, and changelogs
+
 ## GitHub Operations
 
 - PRs require passing CI: typecheck, test, build, lint, changeset check