..but i bought both, it said Diablo and Hellfire. When i click "Hellfire" in the launcher, first nothing happens and 2 seconds later the play-button in the galaxy switches to "play" again.
Can someone help me out ?

You can use DevilutionX to run it, it also has a German translation:
https://github.com/diasurgical/devilutionX#readme

Many thanks, both games are working now !
Do you have a hint, where i could put the DevilutionX-file to reach it ? Is there a way maybe from Galaxy, i could start it ?

It can be run from anywhere, you can also create a shortcut for it in Windows.

Maybe this will work for Galaxy: https://www.gog.com/forum/general/how_to_add_games_manually_to_gog_galaxy

On the link there was this hint:

1. Open Galaxy
2. Go to the page of the game you want to import
3. Select "More" -> "Manage" -> "Import Folder"
4. Click on "Select Folder"
5. Navigate to the folder you installed the game in
6. Click "OK"

So i go to the diablo page in my galaxy, but i don't find "more" there. Maybe things have changed meanwhile because of updates.

i had some issues running hellfire myself. gog for some reason forgot to update some of the hellfire files. you have to go to the dx folder and copy them over manually. think those files is:ddraw.dll, standard.snp and storm.dll. you might also have to copy over the smackw32.dll for the movies to work properly.