SOLVED: Localisation of Track Selection

[LeutnantNomad]

Ok by pure coincidence I had a look into the settings of my Jellyfin container in the Container Manager of DSM (Synology NAS) and found the environment variables for locale "LC_ALL", "LANG" and "LANGUAGE". An explanation of those can be found here. They have been set to en_US, which led to the faulty translation of only the track title. Apparently, the track title generation relies on system locale, while the whole GUI is controlled by the preferred language in Dashboard > General or the profile preference in Settings > Display, so to speak variables in the program.

For the sake of completeness, my environment variables for a proper German setup of Jellyfin:
LC_ALL='de_DE.UTF-8'
LANG='de_DE.UTF-8'
LANGUAGE='de:en'
Note the UTF-8 encoding which is most likely necessary for our Umlaute or other fancy characters. Also, LANGUAGE has the fallback option for English.

Thanks TheDreadPirate for the help anyway!
Hope this helps some other German speaking fellow with OCD.