Hi - on the store page it says that the audio of various languages is included, but in the game I only have the option "English audio". Can anybody help me with changing the audio to German? (I have also tried to do so via GoG Galaxy which did not work)...

If you're using the offline installers, you have to click "Options" before hitting the "Install" button. Take a look at the first two screenshots.

If you're using Galaxy, you have to go to "Settings \ Configure" to change the language. Look at the screenshots 3 and 4.

PS: I know, i have to cleanup my hard drive. :)

Wow - thanks! That was quick! Thank you for the instructions!
Yep - my hard drive would need a cleanup as well ;-)

You're welcome. :)