Common

Common was the trade language of the Bardings, the most ancient race of man. Almost every sentient, civilized being spoke a smattering of it, and most were fluent. While most could not read or write the language, generally, the people of Iosoth were able to converse with each other in this language. Common was little more than a trade language; that is, it was not useful for complicated topics. It was simple and not very expressive as a language. Nearly everyone on the surface of Iosoth spoke Common as a second language, including non-humans. Those in remote areas spoke it badly, but most could get by.

Dialects

Dialects varied from region to region, but generally, communication was possible. Calant was a melodious dialect spoken throughout central Altheria, while Kouroo was a Chultan variant mostly spoken in the southern parts of Iosoth, and Sossal was spoken in the far east of Kuthanion, but there were literally thousands of other local dialects.
The Common spoken throughout Iosoth was properly called Aelfina by linguists and peoples from other continents, such as _______. The purest version of Aelfina was spoken mainly in the western quarter of Iosoth, from Fall Bay in the north to Taime in the south, and from Bellfose in the west to Redwind in the east.
In other realms, "Common" could be altogether different; these were not in fact dialects but different languages. An example was in the Underdark, as the residents there spoke what was known as Undercommon. Inhabitants of different planes also speak different forms of "Common".

Limitations and Advantages

The obvious advantage to Common was that practically every sentient creature could speak it. Even in remote areas of Iosoth like Ajard Island, Kahtl Island, and Anthon, the inhabitants could speak at least enough of it to make themselves understood with a little creative pantomiming. Admittedly, regional accents had a tendency to confuse the issue; natives of distant areas sometimes regarded each others' accents as strange or silly, but they could still understand each other.
However, it also had its disadvantages: not every concept could be easily related to others. Nuances of speech, names, and phrases could be expressed easier in older languages such as Dunarian and Dwarven.

Script

Common used the English alphabet. Elves sometimes wrote Common in Elvish, though some words would have to be adjusted.