Its because they will be communicating in Japanese in the party, if you cannot understand a shout for something you will not unterstand what they are going to do, take Garuda, you can't say "Stand behind this rock and check where she moves for minstral shreak, then move round the rock so its blocks her attack." in auto translate. Because of this the Japanese don't invite many none Japanese speakers because they will need to know what to do, and I have been in PUGs where people do not know what they are doing.

Its just the language barrier nothing else.