By the time that the 2nd movie is due to be set in (I think it's 10 years after the events of the first) Jake should be extremely fluent in Na'vi and would probably only speak in Na'vi when talking to other Na'vi including Neyteri. One question is will the humans who remained still be alive, after-all I'm not sure if they can eat the local foods. The Avatar drivers (inc Norm if his Avatar did survive) could have their consciousness transferred permanently into their Avatars, if that were to happen then they might be expected to only speak Na'vi from then on.
It's always possible that Grace if she returns (her soul is with Eywa and her Avatar AFAIK is still alive to potentially her consciousness could be downloaded into her Avatar), then she would probably only speak in English with other Avatars, humans and possibly with Jake.
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