The chant heard in the Aztec Temple is the same one heard during the ritual in The Night of the Druid's Blood (1966).
The title makes reference to the last Aztec king that reigned at the time of the invasion by the Spanish Conquistadors in the early 16th century.
Unlike most native American languages, Nahuatl, the language of the Aztecs, actually spread in influence during and after European colonization. At the time of these events, it would have been fairly easy to find interpreters. In the present day, it has approximately one and a half million native speakers, more than any other North American language (Navajo comes in second with about a tenth that number).