The program is in English only?
I see a translation file for german - so I think your locale setting is not entirely correct.
/usr/share/locale/de_DE/LC_MESSAGES/grsync.mo
This can happen because KDE may write non-existing locales to /etc/locale.conf.
Ensure your LANG and LANGUAGE is set in /etc/locale.conf
LANGUAGE=de_DE.UTF-8
LANG=de_DE.UTF-8
Alse ensure the file /etc/locale.gen has this line uncommented
de_DE.UTF-8 UTF-8
Then run the locale generation script
sudo locale-gen
Also check the file ~/.config/plasma-localerc
LANGUAGE=de_DE.UTF-8
LANG=de_DE.UTF-8