Most of the animal noises heard in the movie are recycled dinosaur noises from Jurassic Park (1993), notably the T-Rex and the Raptors.
The Na'vi language was created entirely from scratch by linguist Dr. Paul R. Frommer. James Cameron hired him to construct a language that would be easily enunciable for actors to pronounce but would not resemble any human language. Frommer created about 1,000 words. Cameron requested Dr Frommer that the Na'vi language must be entirely new, as it's supposed to be from another planet, and that it should sound "nice" to the audience. Cameron didn't want any additional editing done to their voices and wanted them to sound authentic and not unnatural. Equipped with a vast knowledge of linguistics, Dr Frommer understood what kinds of sounds the Na'vi language would or wouldn't have. Just like "j" and "r" don't exist in Korean, in English, the 'h' sound is unaspirated. In a similar vein, Frommer's conception of the new language totally avoided the sounds of "ch", "th", and "sh". Additionally, Frommer had to decide on the language's syntax -- or rules regarding word order. He came up with his own set of constraints and words to be used in the language of Pandora. The tongue took inspiration from the natural languages of the world to create a totally different spoken form.
James Cameron was convinced that CGI effects had progressed enough to make this film when he saw Gollum in Le Seigneur des anneaux : Les Deux Tours (2002).
This movie took four years to make, from pre-production to release.
James Cameron: His voice is heard over the radio shortly after the Dragon Gunship appears on screen, en route to attack the Na'vi Hometree.
James Cameron: [feet] Close-up of Jake's feet when he moves them around in the soil; close-up of Col. Quaritch's feet in his first scene; slow-mo focus on the wolf-creature's feet as it circles around Jake's avatar, just before he first meets Neytiri.