Kevin Kline had a professor to coach him with the French to speak it as a Frenchman. He studied French during his Jr. High/High school years and a year in college. He didn't learn to speak it until he went to Alliance Française in New York.
As the closing credits roll, Kate (Meg Ryan) asks Luc (Kevin Kline) to sing "that Bobby Darin song" and then begins to actually sing it . . . "Somewhere..." and Luc corrects her to state that it is a French song and begins to sing it in French, "La Mer". That song was originally written by Charles Trenet and then translated into English and recorded by Bobby Darin.
Director Lawrence Kasdan agreed to direct this film project because he had just been involved on a difficult production shoot on Wyatt Earp (1994), would not have to write the screenplay first, and principal photography would involve a nice working holiday for he and his family in France.
Adam Brooks: The writer has a brief role as "Perfect Passenger" (the guy who is taking his shoes off when Kate first gets on the plane to Paris).