Hi Alberto, can you share the link to your website? I will take a look and provide you the CSS to center the flags.
Also, can you clarify the 2nd part of your question in regards to your statement: "change how the other flags are displayed and how to do it". I didn't quite understand what you are wanting to do specifically.
I do know that you won't be able to change the "order" of the flags, although you can show or hide any combination of up to 70 flags on your website. Simply place a check mark next to the flags you want to display and they will show up on your website.
I'll look forward to your reply. Thanks!