Old Roulean is a language used by the traders of the Isparian nations, and as the common language on Dereth.
On Ispar, the nation of Roulea (the equivalent to Earth's Roman Empire) was once a vast empire stretching arcross the whole of The Ironsea. The empire later fell apart, but its language has spread to nearly every corner of Ispar, and was used as a common trading language. When the Aluvians, Gharu'ndim, and Sho were thrown together on Dereth, they adopted this trade tongue as a common language. In real world terms, Old Roulean (also called just Roulean) is the English language.