I can understand the OPs point but if you think this through you went have to ask this question..
This is a fictional universe, depending only on the creator..
Thus the creator can decide to create an all Japanese Universe bound by his own laws..
You see asking why there are no African or American or whatnot kids, it's like asking why there are no green humans, or yellow humans.. The creator of our universe whoever he is, decided that we can only have a limited number of races on Earth.. In similar vein, Kubo Tite can decide, there will only be Japanese in his universe...
Never has he said that his Bleach universe is a replica of our real world.. so you don't expect him to have all races that our universe or Earth has..
More so the use of one language, makes the story telling easy.. I think you should sit back and enjoy the story rather think about the lack of representation of some races..
Cheers