What would a D&D world, where elves and trolls are just as much a part of reality as humanity, look like when it manifests in these other African/Mesoamerican/North American First Nations/Middle Eastern/Asian cultures?
In other words, how would the demihuman and humanoid cultures evolve, vis a vis human cultures, when the various demihuman and humanoid "core" races (esp. dwarves, elves, gnomes, halflings, kobolds, goblins, orcs, hobgoblins, bugbears, gnolls, ogres, and giants) are found, more or less equally distributed, across all continents and regions.
Barring divine intervention, and assuming your scenario of even distribution with human dominance in general, the likeliest scenario is that the various local tribal groups of these races would evolve a culture distinct from but in ways related to that of the dominant human culture. As mentioned, examples of this have been found in numerous published settings.
Hamunaptra from Green Ronin gives a perfect example of traditional D&D demihuman and humanoid races adapting to an "Egyptian-style" culture. Some of the core assumptions of the races were adjusted to fit "Egyptian-style" ideals, but the races were, otherwise, core D&D.
However, if you want to be more "realistic," you should take into account that the traditional D&D demi-human races are very European-centric, cast through the Tolkienesque lens. Other historical cultures had very different spirits and boogeymen, with wholly different assumptions about their nature and relation to humans. Others have mentioned the work it takes to fit this into their own campaigns.
It comes down to the flavor you want for your own campaign setting. If you want the races to have a classic Tolkien high-fantasy feel, even when they are found in a land dominated by an "Aztec-style" human culture, just do it. You should have a good reason for it, though... perhaps the demihumans are immigrants, or the Aztecs invaded centuries ago and destroyed the nascent "German style" human culture, and the demihumans have remained aloof from the new human peoples.
Then, of course, there is the divine intervention mode of cultural diffusion, which I use to explain the ancient human styles of cultures in the Wilderlands of High Adventure. Viking sensibilities rule among the Skandiks because that's the way that Thor and Odin want their followers to live. The Viridians have a culture that is a mish-mash of Persian and Greek and Roman and Celtic because the gods of their (mixed) forefathers wanted their followers to have those cultural elements... and Armadad Bog didn't really care any which way, so over 4,000 years the cultural elements mixed and evolved. Ghinorians have an Egyptian culture because they revere the Egyptian gods. And so forth.
The same can be true for demihumans and humanoids. If you use the classic demihuman and humanoid deities, then their followers could well have their "traditional" cultures because that's what the gods want them to have. Goblins smelt iron and steel in a Copper Age Aztec realm because their gods gave them the secrets. Maybe they in turn will influence the development of the Aztec culture... if their gods let them change.
Really, as with most things, the answer is "their cultures will look however you want them to look." Just remember that, even if the players and the player characters don't know why things are so, there should be a reason behind your decision, whatever way you choose.
