I just changed from owncloud to nextcloud and therefore am very new in this forum.
If I set the default and force languge in config.php to ‘de’ I get a strange mixture of English and German. In the documents tab the left column is given in German but the items in the list show English text (“5 minutes ago” instead of “vor 5 Minuten”). After activating calendar and contact apps contacts are shown with English labels (“Address” instead “Adresse”, “Postal code” instead of “PLZ” and so on). The calendar app has English weekday names and shows event time as “7:00pm” instead of “19:00”. Seems not to be very applicable …