Common, also known as Aenglish, is the official language of the Kingdom of Evenhold and is the most widely spoken language throughout much of central Addar and beyond. Even in places where it's not the primary tongue it's widely understood as a trade or diplomatic language, and most intelligent beings have some familiarity with it.
Like the earlier Latin and Britonnic tongues, Aenglish came to Eamon via rifts that disproportionately opened to points in the British Isles of Earth, with the people who crossed over bringing their native speech, practices, skills, and beliefs with them. Those seeds took root in the cultural soil of Addar and spread, with the terrestrial language ultimately supplanting in many places the earlier common tongues.
Aenglish (originally Æglisc) is much the same as modern English and can be easily understood by Anglphones on Earth, but the language is seasoned with a variety of terms and idioms adopted from other languages including Kurabysh, Thyatian, and Thorass.