The Webstudio app on Windows with an English OS assumes all text to be English and marks every word in other languages as wrong. Notion that uses a similar wrapper gives users an option to turn off spell-checking (second screenshot). Could you have a look whether you can also give us this option?
I have macos / en 2 languages in keyboard setting 3 languages here chrome://settings/languages as Preferred languages but I don't have fr and have no issues with french language spellcheck