Well if you look at Durmstrang for example, they take students from all over the Slavic countries and then also Denmark, Sweden, Finland, etc, etc. Most Southern European students go to Beuxbatons instead of only French. 

Hogwarts just happens to be all of Britain (which is pretty big anyway).

It makes sense that Mahoutokoro would cated to not only Japanese students, but the children of other Asian countries. Especially the southern Asian countries that are island based. :D