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.
When Kate (Meg Ryan) asks Luc (Kevin Kline) are first talking in their room at Cannes, the French song "Verlaine" is playing. The track's lyrics include the wording "les sanglots longues des violons de l'automne blessent mon coeur d'un langueur monotone" which translate into English as "the long sobs of the violins of autumn wound my heart with a monotonous languor". These words are from a poem called "Verlaine", and had been used in 1944 to form the code phrases that alerted the Resistance to the Allied invasion of France, and were depicted in the earlier epic World War II movie The Longest Day (1962).
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.
"You can't leave Paris without seeing the Eiffel Tower." At least twice, Kate misses seeing the tower, once as the car she is in passes the Trocadéro, and next when the tower lights go out at night, just right after she turns her head towards it. Then she sees, reflected in a shop window, the top of what she thinks is the tower, but when she turns to look, the buildings are hiding the entire view. Meanwhile, the window has been opened so that when she turns back, she can't even see the reflection. Finally, when she is leaving Paris, she sees it from the window of the train.
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.