Um... [That's a difficult question] Everyone from home. [Her home world, that is.] Zero, Ichiru-kun, Ai-chan and myself... [So that's four]
Sebastian-san and Lelouch-san can speak Japanese. [They've intentionally done so with her in the past. And Lelouch's friend Suzaku. She's not sure if Euphemia-san can as well or not. Oh. And there's Kazuki and Soushi, too.]
Maybe eight of us or so? There could be more, and those numbers could be a little off. But outside of my family and friends from home, it's not the native language of some of those who are still fluent in it. [And there are still people here she's not especially close with, so they may or may not be fluent in it as well.]