IMPORTANT!!! SERBIA WAS NEVER ADDED IN AGAR. IO, THIS IS FAKE RUMOR SOMEONE MADE ON WIKI. THIS SKIN WAS ONLY AVAILABLE IN AGARIOMODS.
Serbia was one of the country skins in Agar.io. It can be used by typing "Serbia" in the nickname box before the game begins. Then the player's cell will become the flag of Serbia. For random reasons, Agar.io removed this skin from the game. You can still use this by Agar.io mods.