I just installed Firefox in French, and setup up LanguageTools, which the process was in French, I think it detected the language of my browser, but when I go to the same note in my Nextcloud, which use .md (Markdown), LanguageTools have the same behaviour.
I see there’s also English text in your screenshot (“Take note of your ideas”). LT can get confused if there’s more than one language per text. We’re working on a fix for that, but it will still take some time.