Thanks for your answer. When I switch to german language as spoken language, and play the game, the spoken language is still english. the subtitles are german, that is working.
I attached an Image of the game settings, beside the item "spoken language" "deutsch" you can see a button "download language pack".
I clicked on this button, my firefox browser opens a link and a file of 1,0 GB was downloaded. My Win10 defines the downloaded file as "dreamfall chapters setup", but could not open the file and install the language pack.
I did not use steam to download the game, I installed the game with the dvd i got from backerskit.