Nahuatl is an indigenous language of Mexico, with about 1.7 million modern-day speakers. It is an agglutinative, polysynthetic language, which is to say its words are formed from concatenating lots and lots of morphemes to the point that they can get really long. It forms part of the Mesoamerican language area. As the language of the Aztecs, it was a widely-spoken language in Central Mexico in the centuries before and after Spanish conquest, before gradually giving way in most places to Spanish. English has a number of loanwords that have ultimately come from Nahuatl (via Spanish), particularly ones referring to food like chilli, chocolate and tomato or animals like coyote and axolotl.
The modern Nahuatl-speaking population is quite fractured; speakers generally prefer to just use Spanish with anyone who speaks a different dialect of Nahuatl from their own, no matter how similar. And apparently most Nahuatl dialects are similar enough for there to be a high degree of mutual intelligibility, but speakers are not used to having to adjust to other dialects and, really, don’t want to.
Revitalisation projects have generally revolved around the Mexican authorities trying to teach literacy in Classical Nahuatl, or a modern continuation of such. Classical Nahuatl refers to the written form of the language in the 16th and 17th centuries, after it had started to be written in the Latin alphabet. However, it is pretty unpopular among many Nahuatl speakers, because it’s not a dialect anyone actually speaks these days, but instead a “fossilised” dialect from the colonial era. They also resent that when non-Nahua people want to learn Nahuatl, they always want to learn Classical Nahuatl, as the “sexy” form of Nahuatl that the Aztecs spoke.
All modern dialects have been influenced to varying degrees by Spanish. For example, while the predominant word order in Classical Nahuatl was V-S-O, modern varieties are predominantly S-V-O (but word order tends to be more flexible than in, say, English or Spanish, so this is more of a general tendency). Modern Nahuatl also tends to make use of a lot of Spanish words, including conjunctions like pero or porke. Spanish prepositions also compete with the traditional Nahuatl postpostitions, and when the Nahuatl postpositions are used, they’re often positioned as prepositions, that is before instead of after the noun they modify.
Modern Nahuatl speakers mostly live in poorer, rural areas. About half are involved in agriculture, and 60% either receive no wages or get less than the minimum wage. About 10–15% are monolingual.