The voice-language in the game is English, and it seems that you cannot change it in this version of gog-games (03/2021). But you can change the language of undertitles, if you want. While installing the game, there comes a menu where you can select the language. Change the name of language from "English" to another one, and you have the undertitels in this language. Start the game and acitivate "yes" in the menu "undertitles". Hope it works for you.