This article is from the Mongolia FAQ, by Oliver Corff with numerous contributions by others.
Virtually all citizens of Mongolia proper speak Mongolian. Some do not
because they are either of Kazakh or other ethnic origin. Not all
ethnic Mongols in Southern Mongolia do speak Mongol, many of them have
switched to Chinese. Similar phenomena can be observed in Buryatia
where many inhabitants speak Russian. The minor communities scattered
over China (Dongxiang (cf. article in Infosystem Mongolei), Dagur,
Eastern Yugur, Tuzu, Bao'an etc.) and Afghanistan (Moghol) speak some
very old varieties of Mongolian which have developed into proper
languages in their own right. Some of these languages are not well
documented. The Kalmyks speak a form of Mongolian known as Kalmyk
which even developed its own modified form of writing known as ``Tod''
or ``clear'' writing because it identifies vowels and some consonants
(k/g, t/d) in an unambiguous manner.