In dom0, install the German language. In a dom0 terminal, execute sudo qubes-dom0-update langpacks-de.
Then reboot.
Before logging in, the task panel will show - usually far to the right - the current language, e.g. C.UTF-8. Clicking on this value will open a menu where you can select the GUI language.
This need only be done once; the selected language survives logging out and reboot.