Since the Classic HD version of the game does not support any German at all and the origial did support German I tried the still working widescreen add on. However after applying it several menus in the game changed to English. Ist there a German version of the widescrteen add-on or any other possibility to run the game (original or classic HD) with German language?

You're using this version, right? Which menus does it change?

Yes, this one. In example the mane menu buttons do change along with several buttons like "sleep", "tasks", "clothes".

You're right, that's very annoying. The moddb link page suggests that the mod was made with only the Russian and English versions in mind, so this must be an oversight on the modder's part. I've figured out which files are the problem but I don't know how to properly edit them so I can't fix it for you. :/ Fortunately the mod only appears to affect the ui with all of the other text being unchanged, so it's still almost exclusively German. If you can live with some of the menus using English it should be playable as there is unfortunately no other German version that I'm aware of. It might help if you think of it as the translator trying to be hip by sprinkling their work with some unnecessary English. ;)

It's not that much of a problem. I do understand English and I also still have the not modded German version if I should really want to play completely in German. It was just that I thought it was a bit sad to have such a great mod but still not being perfect with the German version and that it MIGHT be that there is another version or a trick to change it. But thank you very much for looking into the issue for me. :)

You're welcome. :) I was just hoping to find a perfect fix for it since the HD version unfortunately didn't get the German translation, especially since it's pretty good. There might still be a trick that I'm overlooking, but it would probably take the original modder making some changes to their mod to fix this.