Looks to me like WordPress doesn’t enable UTF-8 mode when it runs the regex.
https://github.com/WordPress/WordPress/blob/5.2.1/wp-includes/rest-api/class-wp-rest-server.php#L837
In PHP, matching multibyte characters requires the u flag. (The code above just adds the case-insensitive flag). So although some strings will still match (like “Département”), they’re not matching as whole Unicode characters. For example the following pattern matches when it appears is shouldn’t.
preg_match('@^[a-zA-ZÀ-ž][\\wÀ-ÿ]+$@i','£');
// returns 1
The “£” is matched, because it’s made of two bytes (C2 A3). With UTF-8 mode enabled it doesn’t match at the start of the string, because as a character (00A3) it’s greater than “z” (007A) but less than “À” (00C0) in the first character range.
preg_match('@^[a-zA-ZÀ-ž]@iu','£');
// returns 0
So without seeing a specific input string that you’re failing to match, this might explain why it’s not working. Probably you don’t need to be specifying such a specific pattern and can perhaps match any byte with .+ following a safely matched prefix.