The Common Tongue or Common Tongue of Westeros is a language commonly spoken in the Seven Kingdoms of Westeros. There are regional accents, as "Pyp had travelled the Seven Kingdoms with a mummers' troupe, and bragged that he could tell what you were and where you'd been born just from the sound of your voice", but there is no lack of comprehensibility, in any region. Some regional accents can be geographically limited to a single city, as King's Landing.
While some free folk who live beyond the Wall understand the Common Tongue, many clans instead speak local languages. The Thenns, for instance, speak the Old Tongue and can only speak a few words from the Common Tongue.