Hi rosa :) ,
if you just want your country flag to show up in your profile do as the other commenters have suggested 
*If you want a flag as part of your nick there are a few ways to add a flag emoji to your  name 
-you could google "list of flag emojis" and then highlight and copy the flag emoji you want and then paste as part of your nick when making a new nick or changing your old nick 
:)