[Murph]

So, in other words, if you want to change the default country, open the quick missions for each map in FMB and find the planes, select them and change their nationality. Save the changes to the mission, and "hey presto" next time you open that map in QMB, it should show your newly selected nation as default for that map.
Remember that in order to work properly, the mission must include a minimum of four flights for each side, so you must change the nationality of all four flights per side.