This is my FIRST attempt with backend code, so be gentle.
Flags icon are offered by https://catamphetamine.gitlab.io/country-flag-icons/3x2/ . Visit the link to see all available flags and country code.
A Flarum extension. Add country flags attribute to Flarum Users
Install with composer:
composer require justoverclock/country-flags:"*"
composer update justoverclock/country-flags:"*"
php flarum migrate
php flarum cache:clear