Generally maps I create are not set in periods where flags are the primary form of denoting national origin. I have created maps using Japanese kammon (heraldry), as well as European heraldric symbols - shields mostly, but most maps I leave national symbols out altogether. Consider the idea of "nations" and national flags didn't really exist until the rise of modern republics at the end of the 18th century. For maps set in the late 18th century forward, flags seem the ideal symbol for nation.
Nice flag, though.