That’s very strange. Looking at the code the test that results in that message is explicitly looking for the language setting of the site to be ‘en_US’ or just ‘en’. It is hard to know what is going on there without being able to interrogate your database directly. But it does NOT look at downloaded language files – only the WP language setting in the database.
One of the worse things that the WordPress team ever did was to start a process that ended up using incorrect language codes. The official language code for French is simply ‘fr’. ‘fr_FR’ is a nonsense code made up by WordPress. The SP Translation site – interestingly also a WordPress owned system – actually uses the correct codes so all French project files (intended to be used in France itself) are simply ‘fr’ as they should be.
This is beyond our control and it means that the MO files generated by the website often need to have their filename adjusted to match the incorrect WordPress language codes.
The Language panel in the forum admin will try and change these to what the user needs them to be except in your case, of course, it thinks you are running in English. While I know there are one or two issues with the system in 5.5.3 (mainly to do with identifying some of our plugins), mistaking your site to be English is not one of them that we know of and not one we can replicare.
This does mean that a manual download from the translation site is probably the way to go until at least you can try the next SP update at the end of this month
As Ike said above – the bulk of the translations that you see on the forum screen will come from the theme MO file but thete is an easy way to determine of the core MO file is doing it;s job. SImply open the ‘About’ popup (from the link on the footer) and you should see the terms in there in your own language using the core SP language MO file.