All good questions.
The more and more thought I put into this, the less and sure I am about having this part in my comic. It's a bit part, just a joke about something related to the character speaking Japanese. Another character is also Japanese and even another character might be bilingual, but that all doesn't do much to service the plot. For something like this, I know most will say there has to be a reason behind it, whether it's plot related or character development.
They are bilingual. They speak mostly Japanese at home and they were taught by their father.
English is their second language but they speak both just fine.
In a general sense, the character in question has assimilated into the culture they now live in, which is now "American" (I don't really give a set setting, but it's pretty much america) They're an immigrant's kid. They have friends who speak english and they don't really feel isolated from them.
Lastly, having the character in question doesn't weigh that heavy on the plot. It's more for character development. The point of the story isn't to inform, it's to entertain. What I'm really trying to say is that this character can speak another language and that's pretty much it.