If this is a medieval sim...
"English" armies of the medieval period could include: Scots, Irish, Flemish/Dutch, Norman/Angevin/Breton French, Cornish, Welsh + a multitude of ethnicities that are no longer distinct. "Mongol" armies of the same period might have included Turkic, Chinese, Korean, Tibetan, Persian, Caucasian, Russian, and any number of the billion other groups who joined their pan-ethnic state. Byzantine and Turkish armies could appear to be almost identical, including elements from as far afield as England and Iraq and anywhere in between (Normans at times even sided with both sides for personal gain). Don't even get me started on the makeup of Italian, Spanish, German armies.
This period was full of private companies, armies, tribal groups for hire. As well as this, the variations on the different feudal systems in place guaranteed that military forces were cosmopolitan. A lord could simultaneously be an owner of lands in Scotland and Germany, and thus form armies including both. Looking at armies as "ethnically" related to the leadership is a very modern way of thinking, and not really appropriate.
Therefore I will spend my time hand crafting a professional force full of Sharpshooters, Fians, Banner knights and Khan's guards, and I will shape them around what ever cheap bulk infantry I collect on the march.