I still think it's weird how so few Japanese players seem to know English. I'm not English myself, and I know a ton of others who aren't but still speak (or atleast write) it fluently. I'm not expecting fluent english from most Japanese players, but enough to make themselves understood really isn't too much to ask for, I think.
I don't think I've had a lot of Japanese players that has rejected a party invite because I don't speak Japanese. Not if they've got their flag up. And I agree that the overall quality of a party usually is better with more japanese players. Maybe they're just more dedicated/played for a longer time, I don't think it has got a lot to do with intelligence.