If you think it's a jolly good idea to have your country flag shown next to your nickname, here is how you can do it:
Click on your profile image (top-right corner on desktop) and select "Settings."
Scroll down to the "Country Code" section, and pick your country from the drop-down list:
That's it. 🙂
P.S. I'm hoping that @Amadex will manage to add the Yugoslav flag. 🙂