Does look to be the case.

nyan.cat has a language picker that includes Catalan, even though it just changes the page title.

Your browser (if you're using one of the "usual ones") doesn't really do much with the response's status code if it doesn't match a few specific ones for redirecting/caching/protocol shenanigans.

Anything in the 4XX range is going to be treated as just a regular ol' response, just like 404. (You could serve an entire site with all responses set to status=404, and be fine... other than probably never getting any cache hits) If you don't include a body in the response, the browser might sub in it's own error page, but it will just communicate that the user agent made a bad request.