All speaking peoples, including the humans of various lands, possess a native tongue. In addition, all humans and many nonhumans speak Common as a second language. Common grew from a kind of pidgin Ambvrria and is most closely related to that language, but it is far simpler and less expressive. Nuances of speech, naming, and phrasing are better conveyed in the older, more mature languages, since Common is little more than a trade language.
The great advantage of Common, of course, is its prevalence. Everybody speaks Common well enough to get by in any but the most esoteric conversations. Even in remote areas such as Northerond, just about everybody knows enough Common to speak it badly. They might need to point or pantomime in a pinch, but they can make themselves understood. Natives of widely separated areas are likely to regard each other’s accents as strange or even silly, but they still understand each other.