You're using a 1-page thread (which by reading all the responses, is NOT supported by everyone who responded) is hardly very credible evidence to suggest the ENTIRE Japanese community feels the same way as the extremely tiny minority who apparently posted that message.
But, to humor you, reading the thread, it's from someone who didn't like language barriers when it was necessary to communicate what to do in dungeons to people who are unfamiliar with it, ie newbies. As my tip covered, it is only really recommended to join other language DF groups when you are quite familiar already with that duty, and barely any instruction is necessary. Furthermore, pre-communicating to everyone that there is a language barrier and to be patient with you while translating, has ALWAYS, in my experience, netted positive results, positive responses, and even very friendly attitudes.
So, judging by my own experience which is completely contradictory to the SMALL minority you quoted, I think it is quite alright to join other language duties. It has YET to be a problem for me or anyone I've played with, and none of them have ever expressed any negativity over it. Matter of fact, I've accomplished far more in JP groups than English-only groups, and I can't recall any of those JP groups expressing negativity whatsoever towards me. However, I've been in many major FAIL english groups, and lots of blaming goes around - almost always directed at the language barrier, rather than the real problem. Point here is: language barriers are rarely the problem. It's either the people themselves, or how the instructions are given.
Edit: And lastly, it's pretty obvious why SE doesn't totally restrict languages from inter-mingling: they want to promote the world to play together, not segregate based on languages. With an auto-translator built in, that much is obvious.