Common

The common language is a mix of various other languages, from human slang to elven poetry and some draconic curse. It is rather fluid, and easy to pronounce for dragon and man alike. Due to the various race that originated it, le language do keep some rather guttural sound for some expression, mainly curses. Any sound can have two-tone, making for varied meanings with words that may sound similar to strangers.

History

While not the most simple, the language evolution made it the most popular over the year, taking on the name of common language. Originating in tana's empire, the language was formed after the great reunion, when the conflict stopped, the people of the empire had to find a way to talk with each other. The humans, adaptative as they are, and short-lived, took a few generations to adapt, but with this adaptation, this language, mixing the root of the various culture in the empire, appeared.
Tana's empire has been a trading and economic influence over the world, this language slowly spread, it is far from been yet the common language everywhere despite the name. the secluded dwarves speaking very few word of common for example, or the warrior tribe of the green earth having only a basic notion of common.
The writing of common is much more formal, most folks are unable to read or write anyway, the grammar, and orthography were created academically, and therefore is more formal and follow a strict set of rules. The common tongue keeps on evolving, especially as it spread, it grows, adding new vocabulary to its dictionary, borrowed from the various group speaking it. The regional languages still exist, except maybe for the humans, taking on variation of common, most races kept their tradition mostly in their specific language.
The language, even if widespread, is mainly known orally. Very few peasants know how to write and read, as the education necessary to know those are too expensive for them. Nevertheless, the great nation of Toriel do keep written record of their history, especially since the great extinction wiped most of their knowledge of the past, they know to keep written traces of important event.

Common in Politics

Due to the nature of politic to exchange with other country, it became normal to use common for politics, even for local affair. And it is badly perceived to not use common in this field of operation.