Les Demoiselles de Rochefort was the second French musical I'd ever seen. Like Les Parapluies de Cherbourg, it stars Catherine Deneuve but it's less of an opera and more of a musical. I was surprised to see George Chakiris (Bernardo from West Side Story) and Gene Kelly speaking and singing in French, but found upon further inspection that they had been dubbed.
My favorite song was "La Chanson de Maxence." Maxence is a sailor and a painter pining away for his dream woman, aka Catherine Deneuve. P. and S. thought he looked like Draco Malfoy with his blonde hair. The English version is called "You Must Believe in Spring."