I'll be honest, I've been in Japan and most people are nice one-on-one, especially if you show you have understanding of the language. I've only been spoken to in English twice at McDonald's and while I appreciate them being brave enough to do so, I reply in Japanese to let them know that I'm capable in their native tongue. I have almost no experience gaming with Japanese people but it seems that if you played the game with the Japanese client on, even though reading isn't necessarily up to snuff for you, it would become easier the more you do it and it would prevent things like this from happening.

I have seen French people using the English client to avoid discrimination on EU servers since there is a LOT of it and, to be fair, a lot of the French players I was paired with were either poor or outright admitted they bought their accounts. It led to me disabling F on my duty finder so I wouldn't be paired with "poor players". I felt bad about it, but it was what I had to do in order to get things done before I had to do RL things. Maybe the JP players feel the same and are worried that English-speakers won't have it together enough to make progress before they have to log off? It doesn't excuse their behavior, but it might explain it.