Common

Common is also known as Old Asterian. It was taught when Ragnar the Blessed ordered Speakers and Priest out into the lands to learn and teach all they came into contact with. Slowly, but surely, this process allowed for a language to spread. It wasn't until the 1300's that it was discovered such an unintentional event occurred, when the Asterians and Teallachians met with the Rumerlians and learned they understood the Asterian Tongue.
Common is refered to as such by a vast majority of nations, including Asteria. Asteria calls the old language Common and their more modern tongue Asterian.
The vast spread of Common also ensures it's culture and adaptation are widespread as well.