Old Oeridian
Old Oeridian was originally believed to be almost a closed language until Revort Leyhar exploded this myth. He based his case on the obvious fact that such a tongue could not remain free of outside influences, as was once claimed, when those who spoke it were the most widely disseminated and intermixed group throughout the Flanaess.
Geographical Distribution
As a written language, Old Oeridian is used almost exclusively by many everyday scribes (lawyers, clerks, and the like), in part, as a way of maintaining a monopoly on literacy (by excluding mere speakers and writers of Common), and thus their gainful employment. Exalted sages use yet more obscure scripts for the same reasons. Most major archives and libraries have a wealth of materials written in Old Oeridian.
Morphology
Though it forms the major basis of the Common tongue, as a result of its closed nature Old Oeridian's linguistic components are unique and translation into any language except Common is all but impossible.