No idea, it depend on whether they think it would change the campaign’s gameplay or feeling too much. For instance in the 6th Attila mission only the blue player has been changed to Italian, all other enemies have been kept as their OG faction, because changing them all to Italian would make the mission more motonous.
In Barbarossa campaign Mongol merceneries to Cumans. There are a lot of examples, for example one of the battle of the conquerors. There were Turks instead of Berbers.