postSaveLocale() stored language_code from user input with no allow-list validation, and it later flows into Language::setLocale()/load() where the locale segment is require()'d. An authenticated config-grant account could store a relative path (e.g. ../../public/uploads) and, combined with a planted file in public/uploads/, achieve unauthenticated RCE on the next request.
Validate the submitted language against array_keys(get_languages()) before storing, and harden languageExists() to reject path separators and dot-dot sequences. Adds regression tests.