I was waiting to see an answer or two myself, as I have not encountered any Tlazitlan stuff in my reading of Conan stories. Note, however, that I haven't read any of the recent novels, as my only attempt (a Steve Perry offering) left me cold.
From the name Tlazitlan, however, we can infer that they are based on the indigenous Aztec people of what is now Mexico, liberally combined with the Incas of Peru. Expect them to worship dark and strange gods, practice human sacrifice on a grand scale, and build step pyramids.