^ She has been singing in Korea longer than in Japan.
She trained as a kid since she was 11, debuted with ID Peace B in 2000, and then a multilanguage mini album...and this was all in Korea.
As for her ~style~ influencing one over the other.
BoA has always been pretty consistent in her personal styles and preferences as a musician.
Her Korean music definitely influences her Japanese music, she literally has a history of releasing Japanese versions of Korean songs (and vice versa).
Based on her history as a writer and composer I would say she is most comfortable creating midtempos no matter what the language, so I think the tracks we have heard thus far are definitely in line with that.