Common

Common is similar to the language spoken across the planes, the original human language of Sigil.

It's ubiquity throughout the planes mean it can often seem like a default language, although many cultures fight against it's ever encroaching smothering of their mother tongues. Creatures who speak only common (humans, for the most part) often seem to struggle with the concept of foreign languages, preferring to speak Common louder and slower instead of learning any of the language of their hosts.