We would need an ISO-639 code for the language, and ideally get it into ICU databases. Unfortunately, in 2012 Na'vi was denied an ISO code for lack of existing litterature, which puts us in a kind of chicken and egg problem.
It seems we could use art-nav or art-nvi (with "art" being the code for artificial languages). We will still need some hack to make it selectable in the locale preflet, however.
