Mala'kak, a constructed language close to Proto-Indo-European.
The fable is called Schleicher’s Fable. It's a document dating back to 1868, and does exist outside of the film universe. It was written by one August Schleicher in a modernly constructed approximation to Proto-Indo-European, the real precursor of all Indo-European languages. An English translation runs roughly as follows:
[On a hill,] a sheep that had no wool saw horses, one of them pulling a heavy wagon, one carrying a big load, and one carrying a man quickly. The sheep said to the horses: "My heart pains me, seeing a man driving horses." The horses said: "Listen, sheep, our hearts pain us when we see this: a man, the master, makes the wool of the sheep into a warm garment for himself. And the sheep has no wool." Having heard this, the sheep fled into the plain.
This is of course much longer than what is actually used in the film, which is only the first few words of the fable. According to this article (possible spoilers!):
Prometheus star Michael Fassbender spent 17 hours reciting Schleicher’s Fable, committing it to memory, only to have Ridley Scott use only the first line in the film. Talk about dedication!
The language used in the film is not exactly Proto-Indo-European: you can see from Schleicher's original 1868 text that it's not identical to the words in your screenshot. It's another constructed language called Mala'kak, which has its own website (http://www.malakak.com). Particularly relevant are this page on the nature and construction of this language and its relationship to Proto-Indo-European, and this page on Schleicher's fable in Mala'kak as compared to the original Proto-Indo-European.