Oh gods I love this film so much...
Fechez la vache!
Funny story! So my dad works at an international company that draws employees from all over the world. It's not too uncommon for someone from another country to be transferred into the US for a few years, so my dad naturally built up a good collection of foreign friends, including one very good friend who is French. So he was chatting with his friend and a couple of his other French coworkers, and he decides to try and impress them with his nonexistent French-speaking abilities. He waits for an appropriate moment in the conversation and says, very loudly and confidently "Fechez la vache!" and his coworkers look at him like he grew an extra head.
Apparently, "Fechez" is not a real French word