There is a base language called Nuatla. It was spoken by the Aztecs as the first Spanish who visited them. The Spanish recorded it, and unfortunately their spelling is often used for the names of Aztec deities. Why unfortunately? Well English and Spanish do not always pronounce things the same.
Tthe Spanish also destroyed most of the glyphs (pictured sounds) that constituted the Aztec's alphabet but the priests also preserved a few of their deer skin manuscripts and so we have some of the original 'sound symbols' of the Aztecs. See Aztec Writing for more details.
Spelling of Quetzacoatl in English is the Spanish spelling. QUE is not like in 'question' but like in the Spanish "Que passa?" which sounds like "KAY PASSA". (Actually it sounds close to "kuh passa.")
Pronunciation Guide Here
Thanks to Michael Patrick Simpson for this resource.