What does Blanche say to Mitch in French? What is the significance of the French?

Blanche asks him in French if he would like to sleep with her and then says in English that it's a damned good thing that he doesn't understand French. Blanche of course is being passive aggressive with Mitch, at least in her mind. She is manipulating Mitch which is what Blanche does well. She is also asserting her intelligence over his.