There was a reasonably long thread in the Japanese forums that was specifically how being paired with English players in end-game content was making a lot of J players just up and drop duty/immediate kick anyone who were marked as E/did not respond in Japanese.

One problem they laid out was that strategies differ between J and E. Just because you know the dungeon (on end-game content) doesn't mean your strategy 100% meshes with everyone else in the group.