There is a standard language, known simply as common or the Common Tongue, that all humans, dwarves, elves, half-elves and muls speak.
It is important to note that halflings and thri-kreen do not speak common. It is strongly recommended, however, that player character halflings and thri-kreen use one of their proficiency slots to obtain the common tongue.
Sources[]
- Dark Sun Boxed Set. 2nd Edition. TSR Inc.