Country flag automatic registered user

Good evening guys, I found a problem, I would like to add for each user who signs up a flag of his nation in the preview that is in the category sections. Surely I will have to enter in ftp to add some string of code, or in the registration form or I don’t know … my example is this, I know that I will have to add a class to the one I will post here, but it must automatically exit the flag, every time a user chooses the nation in the registration.

<div class="cbp-l-caption-title">Dr Maxim</div>   

Example

Thank you very much, I hope someone can teach me this process.