If a lord defects from Battania and winds up working for the Khuzait, then I would just expect him to have a mongolian force as that is his new home... Then again... I also liked how back in Warband certain "evil" lords would have these hodgepodge forces of "mercenaries" rather than a "straight ticket" type force of the lands to which they live. However, I do not like being forced to play the game according to someone elses whims, for me it is fun to mix-match forces and see what happens on the field of battle so that I can play to MY style. It is still a game at its core.
Historical accuracy on some things be damned, it just makes it more wild when some back stabbing lord can show up with his "core" force and a Mongolian mercenary horde riding with him. Mercenaries are nothing new in history as has been said and in a world this tiny where so many cultures are directly exposed to each other you would just EXPECT there to be overlap as they hire back and forth. If this is a problem the old Warband thing of "Swadians do not like fighting Swadians" would work for morale loss.
Forcing cultural problems would just break things to pieces as the AI is not great and the player would wind up ripping their hair out trying to keep control of the map, you would literally be forced to stay within cultural boundaries or maybe just on the fringes as you endlessly battle uprisings. All the while running home to recruit new fodder to march off to the distant front in an endless war of lose/lose attrition. Historically forces were raised locally to keep control, the Romans had their massed auxiliaries to flesh out their core forces from Italy. Also remember thisL Rome conquered the known world culture be damned, it did bite them in the arse HARD down the road but it is historically proven to work in such time periods before nationalism existed. And nationalism is NOT medieval.
The way I figure it though after having looked over the forums the past week... This is all a moot point as TW wont listen to anyone for good or ill and as such will just plug away until releasing it broken as hell. People will mod the ever loving hell out of it to make things work in their own good tastes, etc, etc.