The Switch-Laguage-Menu you're refering to is there to change de language of your back end. Theses languages are limited in number and correspond to the available translations of Novius OS.
You can see all the available languages for Novius OS back end here :
As you can see, German is (unfortunately) unavailable.
You can contribute to Novius OS by adding german translation if you need it, we'll be happy to support you in this effort
And do not hesitate to ask more if needed or if I'm not being really clear.
allthough I set the German languge as the default one during setup, sites aren't in german, nor can I select german from the Switch-Language-Menue.
What is wrong?