Considering how Grace Augustine said that the kids in her school "picked up English faster than I could teach it" they'd probably send one of the young 'uns to learn and translate for them.
This does bring up an interesting point. Could be wrong but I think I heard once that with the trees of souls all around Pandora, the Na'vi are able to access memories from every region on the moon and thus this resulted in a lack of regional slangs and dialects since no group was isolated enough to have their own unique linguistic identity develop. So, learning that not all the skypeople can understand each other would probably be a big revelation for them.
Someone from the Learn Na'vi forums would probably know better than me.