I had a similar thing with my games: I am German, but my boyfriend is UK. Therefore we want to play our games in English, but my whole Google account + Stadia is in German.
I figured out, that it works when you just change the language of your chromecast in the Google Home App, just do the following:
This worked wonderfully for me. Our first two games in Mortal Kombat 11 were in German and then I switched to English by changing the Chromecast language.