Default browser locale used instead of the one set in the profile
NC 21.0.1. Default browser locale: French Locale set in NC for user: English (UK), language English (BE)
However, the browser default locale is used:

That's odd. The js code call into OC.L10N.translate. That server API should use the correct language/locale.
Actually, it's the whole app which uses the browser's locale.
Status app, weather app, dashboard app have the same problem. Calendar, discourse notifications, deck, bookmarks, news, activities don't have the problem.
Are you still experiencing this?
Status app, weather app, dashboard app have the same problem.
Made me think of nextcloud/server#49988, but it's not quite the same behavior I'd expect.
Default browser locale: French Locale set in NC for user: English (UK), language English (BE)
I tried on a current NC 31.0.2, and I with the settings, I only see English text (as it is supposed to be), except for the login page (where it can't know the user specific settings yet).
So bug seems to be fixed in the meantime.