You can try to open "Terminal" App,
Enter your Admin password and then "Press enter" (You won't see your password and it's normal)
Then choose your language > you'll have a list of languages with numbers > just look for french (number two I Think)
"Press 2, and Enter"
Then restart you Mac and see
You will have to make a back up first
Use Time Machine to back up or restore your Mac - Apple Support
And then Reinstall macOS :
How to reinstall macOS - Apple Support
> Start with Mac with Cmd+R pressed and then click on "Reinstall macOS".
This manipulation won't erase your datas.