The Navi Language was invented for the Avatar movie by American communications professor Paul R. Frommer at the University of Southern California. This is a four minute radio interview from 2009 and it’s very interesting if you care to listen.
As stated this interview was from 2009. Since then it has grown in popularity. It took him four years to create it and with the creation of the video game, the vocabulary has increased. Here’s a tube clip showing it off. It’s not a short clip and it’s a bit technical, just to warn you.