Nope, it's not any of those films. Only
La grande illusion is about the French, anyways, and I don't think it's that one. There aren't very many films about the French in World War I.
Paths of Glory is one (as noted by
Lightwizard), but that doesn't have a scene like the one
Polarbear describes.
Un long dimanche de fiançailles, with Audrey Tautou, is another, but that ain't it either. It might be
Joyeux Noël, which is about the Christmas truce in 1914 -- I haven't seen that movie.
It sounds almost like a scene out of Henri Barbusse's
La feu (English title:
Under Fire), but I don't know if there has ever been a movie adaptation of that book.
There are 909 titles under the
keyword "WWI" in the Internet Movie Database. Maybe one of those titles will spark a recollection,
Polarbear.