Edit: Just in case anyone wonders about the downloadable voice packs in gog galaxy etc. You do not need this for a different voice pack to work. It is sufficient to go to the game, load up the game settings - not sound settings - and select the game voice from there.
Maybe this saves some people time trying to reinstall via gog galaxy/offline installer. gog galaxy even tells you that the game is damaged, when you try to install the voice pack. way to go is the game itself - game settings, not sound settings.

Speaking of language, are NPC supposed to talk half your language and half german?
(in my case english/german)...an effect completely unknown to someone playing this in german...

Yup, this is normal. Kuttenberg was a city with much german speaking population. Even the citys name is de facto german.

Even the word "Groschen" was sometimes used where I grew up as a synonym for coins

The bug still exists and I can’t tell, if the voice pack is damaged or the error message is wrong. I tried to re-download the pack, since I didn’t hear any German dub for the horse race content.