Downloading the standalone installer weighs about 24 GB already. For the german version, an additional download of 3,9 GB is necessary. So, first download the english version (although you don't want to play in english), then patch it to the german version.
I don't think that's customer friendly! Further more, the install process will also take longer. Are there reasons for this?

Does the installation via Galaxy also make this detour?

If your GOG language is German you automaticly get the german version of the game.