B can't speak English, which means B is not English, so B is French or German. "Everyone can speak one of the other three languages besides his own language", so B should be able to speak German and Japanese. None of the four people can speak both Japanese and French. It means that B is German and can speak German.

3. Dialogue between A and C, and B can translate for them. A and C should be able to speak German. But A can speak Japanese. None of these four people can talk in Japanese and French at the same time. So A can speak Japanese and German.

Party A and Party B confirm that Party B can be British or French.

5, "B, C, D can't find the same language", B can speak German, C can speak French or German, then D should be British and can speak Japanese and English.

C is French and can speak English.