Of course, "Genlan" is any language spoken by the Gens out-Territory, wherever in the world the Sime Territory might be. In North America, where most of the stories and novels are set, the "English" spoken would not sound like what Ancients in the region speak today. Likewise in the area that is now Mexico, the surrounding Gens would speak some version of Mexican Spanish derived from what is spoken there today.
After the collapse of our Ancient Civilization, communications go back to the condition of the Middle Ages or worse, where nobody ever hears anyone speak who was born or raised more than maybe 50 "miles" away. Dialects drift rapidly in both Simelan and the surrounding Gen languages, and languages continue to evolve.