Aaaah, this topic again.

I rarely see JP only parties, and only once I was really not allowed to join when I was using Auto-translate and reading what they said in google translate (I'm not using that to translate from English to Japanese though, because I know just how horrible it must be for the Japanese players to read that).
This only applies for grind parties though, because I totally understand why nobody would want to do Ifrit with somebody who wouldn't understand a word

And about the "japanese don't like when you go afk all the time or say brb for all kinds of stuff"-answer:
That's totally not true.
I've played with some extremely nice Japanese players who were all okay with me being stupid and getting arrows from Ul'dah instead of the next camp, or with me DC'ing about every 10 minutes.
Actually, I met way more NA or EU parties who kicked me just because I had DC once.
Like "Uh, it's too much of an effort to reinvite you.."

Btw, just so you know, people don't act that way just because they are non-japanese.
Impolite players act that way, and please don't equate "non-japanese" with "impolite" >.>