Mizuki/Moon is from Kanto which is, of course, based on Japan's Kanto region. The others are from Alola which is based on Hawaii so it makes sense that she's the only one who knows Japanese.
^I didn't mean where physically - I mean the name Mizuki - her only given name in the game is moon right? As in the English word, written in Katakana. So why is she called Mizuki here? Because it has the kanji for moon in it?