@luca_tenji that is a valid response, but wholly unrealistic. you can write a foreign character without being racist or whatever, and even if you can't you can include a foreign character without botching the fish out of water scenario that generally defines such a character trope. it's lazy writing foremost (by deciding to not include why they fit in this setting besides "Japanese ancestry" and thus they speak the language) and then cliched and uninteresting secondly, as they don't tread any new ground. why not include a foreign exchange student who had no background in Japanese? Tsurezure Children did it with a (practically) tertiary character to great effect and even made the language barrier a point of comedy instead of a barrier easily overcome.
I apologize for the rant, it's just as a (poor) student of languages and someone appreciative of diversity I find it inane to make every foreign character somehow also Japanese so as not to upset the balance. if you can't write a foreign character, don't include them.