When creating the movie Avatar, James Cameron turned to a USC linguistics professor named Paul Frommer to create the language of the Na’vi tribe of aliens inhabiting the planet Pandora.
 
There are several resources that cover the Na’vi language. The LanguageLog has a great overview of the Na’vi language. There is also a Learn Na’vi Language wiki.

Frommer talks about the making of the Na’vi language and explains the main perils as follows:

“If you allow everything and the kitchen sink, you get a mishmash, it sounds like gibberish,” Frommer said. “An analogy is cooking and deciding how you are going to spice up a certain dish. If you put everything you have on the shelf, you get a mess. If you are judicious you get something good. In language, sometimes things are defined by the absences.”

The finished product sounds, to some ears, vaguely Polynesian, while others hear the rhythms of African languages in it. “Someone said it sounded German to them, someone else told me Japanese, and I think that’s good. If everyone were saying one single language then it would be bad,”

 Here is More Information on the making of the Na’vi language for the movie Avatar.