Centaurian is the ancient language of the Centaurs. Only a handful of centaurs can read it, and only Foaly can decipher the computer dialect. Foaly codes whatever he does in Centaurian. Also, in the U.K. edition of The Arctic Incident, the code at the bottom of the pages is in Centaurian. The only non-Centaur capable of reading the language and cracking Foaly's computer code was Opal Koboi, and even she needed spy cameras to build a frame of reference.
The link to download the Centaurian font is here : http://artemisfowl.fangathering.com/media/gnommish-fonts/