France/Japan is a pairing involving the characters of France and Japan. The pairing is commonly referred to as Frapan (made by combining the names France and Japan). Another name for the pairing is Nihonhonhon.
France and Japan do not have a big relationship. The pairing is more crack than canon.
Japan has admitted to France (and two others) that he has a preference to 2D girls as opposed to 3D girls, and their canon relationship hasn't really developed from that, sadly.