Using a simple trick that was probably for the dev(s) of this game, you can unlock all the countries easily.
How to Unlock Countries
People have reported that when they entered the game, or pressed a key (like W) they unlocked all the countries.
For me this randomly happened when I revealed the US.
So while on the main menu, in a game or on a character reveal screen, mash your keyboard and then restart the game, and see if that works.
To be honest I dont know how it works but all I know is it happened when I revealed the US and I probably pressed some key on my keyboard so maybe it has something to do with that.
Now if it worked, you can play as all the countries (you still need to unlock them with countrycoins though). Good luck and have fun!