By years of playing with JP players i can saay that only a minority of them would rather playing strictly with JP-speaking players...just like EN, FR and GE-speaking players would too. I think it's all a matter of respect...like using the auto-translate if you don't speak the party language and, if you do, speak in the language the party does.
I very often saw, as example, NA players cursing JP ones because they were speaking JP in PT, as if no JP speak english...just ike i've seen the oposite too...but, again, this is the minority. IMO adding language-specificservers goes against all the globalization path most MMOs are following...IMO it's just the first step for some regional segregation...soon enough you will start having certain languages' servers receiving content before others and soon enough you will have a development scalling regarding servers...just like we see in most region-beased MMOs...because the development is done in a certain language so, obviously, the content for that language will be ready before all others.