- When last we saw Nanase's mother
The whole "speaking Japanese around the house" dealie is inspired by my own trips to the houses of friends who would speak foreign languages amongst themselves. I am not assuming every household that does this has the same reasons Akiko gives here, but heritage, habit and English being a second, third, or whatever language from their perspective all seem realistic to me.
Another good answer is "why not?" Speaking in English is what comes naturally to me, and I doubt that would change even if I learned a second language and moved elsewhere. At least, I don't think I would. I guess I don't know unless I try. I suppose I have no choice but to learn Japanese and move to a Japan for a bit to test my assertion. This could take a while, folks