Actually I base my opinions in a mastery degree in medieval history LOL. I know this have not to be mentioned here, but I could not hold myself this time. LOL
And in the example you give the game already uses it in the form of the mounted archer of the empire. They were first employed by the bizantines as auxiliaries and overtime have been incorporated to the army, then began to use proper armor in Bizantine style and have it's battle tactics incorporated to the army doctrines. This do happen all the time, less so in medieval history. As we know most people in game should not even to be able to speak the same language.
It's even said that the xiites wanted to help the christians to take Jerusalem from the Sunis, and it just don't happened for lack of capacity to communicate in a common language.