Izzy's improved at a lot of languages. She's not exactly fluent in any of them, but between Fiilonof translating and being immersed in so many languages for 6 years she was bound to pick up something. Although French is the only other human language she's conversational in. If she ever does get back to Earth, her family will no longer have a "secret" language to converse in. 

It's funny though. She won't be able to impress anybody with any of the other languages she's picked up. Since almost no one on Earth knows extraterrestrial languages. Well, there is one person who specializes in it. But I'm not sure how Izzy feels about her.