Thanks for writing to us.
You display the flag icon by using custom CSS code. You need to add this CSS code and then adjust the menu item padding as well.
The code output would be like this, please have a look at the screenshot in the secure note.
The purpose of providing the custom CSS to show you how to add CSS code to your site. Writing custom CSS is outside the scope of our theme support. If you need more customization, you need to learn CSS and learn how to use the browser’s element inspector. Please note that we don’t provide any third-party plugin support. It is out of the support scope.
Hope it helps.
Thanks for understanding