To set the flag on your profile, you must update either from the "Career" in-game or by using the !flag command with the avatar bot.

NOTE:
If you already have a flag and change it, it can take several months before it updates on Epic's end.
It's recommended you instead use !flag to change your flag as it's instant.

!flag usage:
!flag <country>
e.g. !flag portugal
(all countries are supported, not just the ones in-game)