The fran?ais added after the preposition de should be a noun; when de is not used, fran?ais is an adjective.
fran?ais can be used as a noun and an adjective. When used as a noun (the first letter is not capitalized), it has only one meaning, which is "French", without the meaning of "France"; when used as an adjective, it has the meaning of "French".
So French movies can only say les films fran?ais