Well my LS is JP, and because I study Japanese, I've earned respect among them. But the general feeling is that English players are bad, so JP won't party with them. For the most part, it's true, but there are plenty of bad JP players too. Also, the general age of JP players is more 20~35 years old, because at this time, they've finished the most important part of their lives, school. Once they've gotten into a good college, they can slack off, but until that point, education is the most important thing in the world. It's sad, but one of the most common suicide reasons, is not getting into college.
Most American players are closer to 13~25(I don't know about EU players,) and most can be pretty immature at that time, so JP's assume all English players are like that.
The Japanese culture is built on respect, and shame. Most of what people have said here is correct too, JP's won't party with English players because of the language problem. Even though they study english, some haven't learned, but some have.
Also, with the Auto translate, not everything translates the way you think it would. Like {back} refers to the Equipment slot, verses {I'm back!} which translates to exactly that, or in Japanese "tadaima!"
Japanese also take their parties seriously. They chat, but it's usually strickly business relating to the party, unless they'res downtime.
I remember once on my WHM, I was in a mostly JP party, the RDM was also an English player. Our MNK and THF pullers got a link, and sadly the BLM got killed from trying to sleep the other spider. I raised her, and she kindly thanked me in English and Japanese, because I had helped to work out conversation between the RDM and I, and the rest of the party and visa versa. She then told the JPs that I was rich, it's not as funny out of context though.
Point of this unnessicarily large post though, is if you're good at what you do, you'll earn respect regardless of language. So try your best to work with what you're given.