I’m going to assume that you are running openHAB on Linux.
According to this posting:
this should be possible.
Open the /etc/default/openhab2 file using vi, nano, or any other text editor and add or update the following options to the EXTRA_JAVA_OPTS variable:
EXTRA_JAVA_OPTS="-Duser.country=DE -Duser.language=de"
Next restart openHAB:
sudo systemctl restart openhab2.service
or simply reboot:
sudo reboot
