The model of Apple Watch used in Switzerland and Germany is the same one.
If Telekom can set it up with their service in Germany, it should indeed work there.
However, the Apple Watch cannot roam outside its carrier's service area, that is, it cannot connect to other carriers when out of the area/country of the carrier providing the cellular service.
If you intend to use it with Telekom in Germany it will not have cellular service when out of the country except through the paired iPhone.