If you switch languages, the date/time format you're using may not have translated months/days-of-the-week, depending on what page you're on.

For example, the user profile page does, while the "last updated" bit on entity pages don't. The date/time format options in the account preferences form also aren't translated.

This would seem to be affecting templates that have been converted to React and are using root/utility/formatUserDate.js rather than lib/MusicBrainz/Server/Plugin/UserDate.pm in TT.