It's important to note that "Aztec" is a broad term. It refers to the dominant culture and people of the Triple Alliance, a group of city-states including Tenochtitlán, Texcoco, and Tlacopan. While the Aztecs spoke Nahuatl, other indigenous groups throughout Mesoamerica also spoke various dialects of Nahuatl.
Nahuatl is still spoken by millions of people today, primarily in Mexico. It is one of the most widely spoken indigenous languages in the Americas.