Realmish is believed to have originated around year 800 S.A. when some major Hordes started to trade instead of warring and it has evolved ever since. Due to its evolution and somewhat widespread usage, the High King chose it to be the official language of the Realm and it has since spread to all of The Realm. It is of course know in varying degrees, as the closer one gets to the major hubs of power and trade the higher the understanding of both it’s spoken and written forms are. The further away from major traffic or interests of note the less it is understood, especially in its written form.
There are actually still tribes who do not use or understand this language at all… for example the crude Archaic rarely seen with other Humans except when enslaved to do the hard work of their superiors…