In Firefox, I'd like to have multiple, or at least two, proofing languages and keep English as main display language. For instance, and as I'm French, the following screenshot of web WhatsApp shows that Firefox corrects my English sentences well but that doesn't work for my native language. I didn't find anything on the net or Firefox's preferences. Maybe this goes with an add-on?
I also have the exact same question for Thunderbird Mail, should I open another question?

