I just searched through my CDs and all I could find was a German version of the Balance of Power Mission-CD. The main game only has English voice and text. After installing the mission CD though a big part of the text in the main game is translated into German. Does somebody know if a "real" German version of the main game was ever released?

Yes, I have the German release of XvT (+BoP) at home. I uploaded the German intros and cutscenes on YT:
https://www.youtube.com/playlist?list=PL93C344F91804F286

So XvT has German subtitles even without installing BoP? Damn - hopefully the German version will arrive at GOG too. Thank you for clearing this - I could not find conclusive informations on the internet.