The Common Tongue in a Central Government

If you have military imperialist adventurers who create a vast empire, it may not be their language that is picked for the lingua franca. They may pick up something that's already established like Quechua was used by the Incan empire. When the Romans took over the Levant, they just left Greek in place. And then I have a reference here to the Dragomon, which is, why can't you remember the Turks? Yeah. The Turkish Empire used them and they were both diplomats and translators and various sort of minor court stuff.