The interface of mediawiki is by default english. It can be adjusted by changing $wgLanguageCode in file LocalSettings.php or probably the 2 other Freedombox*.php files in /etc/mediawiki:
a) which file do I best use for changing in my case to “de” to avoid config questions at upgrades?
b) I suggest to include the change of this field in the user interface Plinth
You should edit /etc/mediawiki/FreedomBoxSettings.php. Looks like you need to run echo 'MessageCache::singleton()->clear()' | php maintenance/eval.php after setting the value according to documentation. I have created an issue for FreedomBox to allow setting this value.
Thanks,
instead of clearing as you explained above, I simple de-activated mediawiki in Plinth and re-activated it again. This also works.