Common

Common was the trade language of Volatia. 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 Volatia were able to converse with each other in this language. Common was not useful for complicated topics. It was simple and not very expressive as a language.
Speakers
Nearly everyone on the surface of Volatia 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. 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 Volatia. 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 Elven and Dwarven.