FR Campaign Setting-page 84 wrote:Language
Common language and culture defines a state just as much as borders, cities, and government do. Each major nonhuman race speaks its own language, and humans seem to generate dozens of languages for no other reason than their lands are so widespread and communications so chancy that language drift occurs over time. Hundreds of human dialects are still spoken daily in Faerun, although Common serves to overcome all but the most overwhelming obstacles to comprehension.
The oldest languages spoken in Faerun are nonhuman in origin. Draconic, the speech of dragons, may be the oldest of all. Giant, Elven, and Dwarven are also ancient tongues. The oldest known human languages date back some three to four thousand years. They come from four main cultural groups—Chondathan, Imaskari, Nar, and Netherese—that had their own languages, some of which survive today in altered forms after centuries of intermingling and trade.
THE COMMON TONGUE
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 Chondathan 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 in the Heartlands speaks Common well enough to get by in any but the most esoteric conversations. Even in remote areas such as Murghom and Samarach, 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.