You saw people that are playing agar with france flag instead of that default colored cell, right?
Well, there isn’t much engineering there. You just need to play agar with a custom profile.
Take a look on the articole about the agar.io skins.
PS: in less words, if you don’t want to read the article about the skins, to play agar with france flag you need just to take the nick france when you start playing. Enjoy!
PS. I wasn’t using the agario cheat 😉