I seem to get asked this question quite a bit. People want to know how to label the people who built and lived in the city of Calixtlahuaca. During the excavations, a group of people from the group "Unión de los Pueblos del Valle de Toluca" visited us and asked if we knew which language was spoken. They represented indigenous people from the western part of the State of Mexico, including speakers of the Mazahua, Matlatzinca, and Otomi languages. It turns out that these three languages, plus Nahuatl, were all present in the Toluca Valley at the time of the Spanish conquest. Here is René García Castro's map of the non-Nahuatl language distribution at that time.
Calixtlahuaca is just above Toluca. It is within the area of Matlatzinca, near its northern border, and also within the area of Otomi (near its southern border). The Mazahua area is not far to the northwest. And we know that Nahuatl was spoken in much of the valley also.
It seems that we cannot answer this question about languages on the basis of present data. My guess is that the city was founded by non-Nahuatl speakers, but by the time the Mexica king Axayacatl conquered the city in the 1470s, there were speakers of all four languages. But that is mostly a guess. Perhaps better data from historical linguistics could help provide a more precise answer. Perhaps if we had better information on the Calixtlahuaca bird, we might be able to narrow down the choice of languages, at least for the ruling dynasty.
This lack of precision is frustrating, but it does not both me unduly. The language(s) spoken and ethnic identity of the inhabitants of Calixtlahuaca are less interesting to me than are their activities and accomplishments.
The map shown above is from García Castro (2000); see his 1999 book for documentation.
García Castro, René
1999 Indios, territorio y poder en la provincia matlatzinca: la negociación del espacio político de los pueblos otomianos, siglos XV-XII. CIESAS, Instituto Nacional de Antropología e Historia, and El Colegio Mexiquense, Mexico City and Toluca.
García Castro, René
2000 Los grupos indígenas del valle de Toluca. Arqueología Mexicana 43:50-55.