I know only one phrase in French (besides je t'aime, which I never had any call for) which is derrier tete. My Irish grandmother use to call of us that, as she pinched our cheeks and smiled lovingly at our innocent little faces. Then I took French in high school. Realizing it sounded very french, I raised my hand in class and asked the teacher what it meant. She immediately asked where I heard it and I told her what my grandmother use to do..yadda yadda.
I had a new nickname in school after that day.
sigh.
I'm sure my grandmother is still laughing her ass off in heaven over it.