Nahuatl is a language spoken in south-central Mexico. Although speakers are few today, it was the administrative language of the Aztec empire, and accordingly was of great interest to the Spanish immigrants who later inherited the administration of Mexico.
Because a knowledge Nahuatl was so important to the early Spanish, it was an object of scholarly concern already by the mid-XVIth century. Accordingly we have a long record of it, and in some ways this makes it one of the most interesting of the indigenous American languages to study. Indeed, as you can see by the picture at the right, Nahuatl can become a consuming passion.)
For learnèd and responsible information about Nahuatl, you may wish to check the Nahuatl Home Page; it is intended for scholars of Nahuatl, but is interesting to others as well (and furthermore has nice graphics). The Summer Institute of Linguistics maintains a page devoted to modern languages of Mexico, including several different dialects of modern Nahuatl, and a web site specifically run by the SIL Mexican branch.
However over the centuries there has been considerable instability in the spelling of Nahuatl. Some common variations:
*-Untypable letters are an occupational hazard of being a linguist. Ordinary mortals hold them in low regard and prefer orthographies which avoid them.