I wanted to know how make Wolfenstein the old bood in french (text and audio). My game starts in english.

Old blood in French is "vieux sang"

/joking.

Go to your collection of games and click on Old Blood. You are going to see something like this: https://imgur.com/a/plJJA4E. Click on "Language" and then choose French. After that, download the game. I'm not sure how to make the game boot up in another language. Have you tried the options in the game? I can't see it myself right now, as I don't have it installed. It's 37 GB, so it would take a while for me to check it out.

Edit: I did some sleuthing and it looks like changing the language without a re-install is prohibitively difficult.