It would be kind of neat though if Willow Smith was actually fluent in Na’vi, and with her and Jaden cut from the same cloth, it is certainly possible.
I figure Na’vi must be near the bottom of total speakers, certainly below Klingon and Elvish, but possibly ahead of Dothraki.
That’s like saying “My mother in law has big hairy feet, can I be in the next Hobbit movie?”
Or like saying “A guy I know has read the Harry Potter books three times, can I be in Fantastic Beasts 2?”