docs: write commit messages in English from now on

CONTRIBUTING (en/de) and README now state that new code comments and commit messages are
written in English.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -147,7 +147,7 @@ cargo test                   # unit and integration tests
 
 CI additionally checks `cargo fmt --check` and `cargo clippy --all-targets -- -D warnings`.
 
-New code comments are written in English; older code still has German comments.
+New code comments and commit messages are written in English; older code and history still contain German.
 
 ### Project structure