The Common Tongue is called that because it the most frequently used language among the western nations of Galador. It is the official language of Haladan, The Wenidarian Empire, Larn, Craynor, the Teas Aradine (though Elven is spoken equally as frequently there), The Thoran Principality and The Contare. It is also spoken by most of the population of each of The Chendorian City-States. Sages refer to the common tongue as New Craynori, but almost no one else uses the lexicon.
In turn, Ancient Craynori evolved from a human dialect so old that noone even knows what it was called. As humans spread across Galador, this language took root words from elven, Dwarven, and many other humanoid languages; humans were the first and most populous race to expand across the continent and meet nearly all of the other humanoid races.