I've downloaded the German installer (which is in fact the same as the English one) and the German language pack, but if I try to install the latter, I'm always getting the message "Installed version is not compatible with the patch".

My Installer is version 1.0.2.13, so I think the language pack was created for an older version. Can anyone from GOG staff please have a look into this matter?

Did you try starting the game? If that message came up at the end of the install, it is probably fine.
There is a known issue that can give an incorrect error; IIRC the language patches were only tested on a clean install of the game, and if the game is started first (or maybe just if the main installer was updated since the language packs were created) there are extra files created or changed, which triggers the compatibility error. The language pack should still be properly installed, though.

Thanks for your reply. I've tried to start the game, and you're right indeed: The language pack works despite the error message. Strange enough, because I've installed the game only once, so it should been a "clean install". Anyway, it works now. Sorry for all the inconvinience and thanks again for your help! :)