Common

Common is the trade language of Faerûn. Almost every sentient civilized being speaks a smattering of it and most are fluent. While most cannot read or write the language, generally, the people of Faerûn will be able to converse with each other in this language.

Dialects
Dialects vary from region to region, but generally, communication should be possible. Calant is a melodious dialect spoken on the Sword Coast, while Kouroo is a Chultan variant, and Skaevrym is spoken in Sossal.

In other realms, "common" may be a different form altogether; these are not in fact dialects, but different languages. An example is in the Underdark, as the residents there speak what is known as Undercommon. Inhabitants of different planes might also speak different forms of "common".

Limitations and Advantages
According to Faerûnian linguists, Common developed out of a pidgin variant of the Jhaamdathan language called Thorass, and thus is most closely related to that language. However, it has its disadvantages; not every concept can be easily related to others. Nuances of speech, names, and phrases can be expressed easier in older languages such as Illuskan and Dwarven.

The obvious advantage to Common is that practically every sentient in the universe can speak it. Even in remote areas of Faerûn like Murghôm, Samarach, and the Great Glacier, the inhabitants can speak at least enough of it to make themselves understood with a little creative pantomiming. Admittedly regional accents have a tendency to confuse the issue; natives of distant areas may regard each others' accents as strange or silly, but they can still understand each other.

History and Alphabet
As has been previously stated, the direct ancestor of Common was a language called Thorass, which an unknown scribe eventually developed a written alphabet for. Thorass is now a dead language (meaning that nobody speaks it on a daily basis anymore), but the alphabet survives, and is now used in languages such as Common, Chessentan, Rashemi and Uluik.

There is a great deal of evidence that Thorass is derived in part from older incarnations of Jotun, the language of giants, along with other unknown (probably Jhaamdathan) influences.