Why does Duo insult me that much????

It's because you used an opening which has a language inside the name. Otherwise, duo is friendly
You are probably right as Duo is angry when I played the English Opening...

He's probably so narssistic in languages that he cannot bear when you try to show him that you know a language when you are not using his app or website
Translate: YOU ARE PLAYING THE FRENCH DEFENSE? IS THIS THE ONLY LANGUAGE YOU KNOW OR WHAT?
Translate: I HAVE NOT READ "LES MISÉRABLES", BUT I ASSUME IT TELLS ABOUT YOUR LEVEL IN CHESS.
I WANTED US TO BE FRIENDS, BUT YOU HAVE NOT STOPPED ENCROACHING MY TERRITORY. THEREFORE, YOU WILL GET WHAT YOU DESERVE.
Duo, please don't punish me:
I don't only learn French...