How to enable country selector in the header

Go Online Store - Customize - Header section. Here you can see the Show country selector field. Tick a checkbox to enable it in the header. As a result, you will see the list of countries at the top.

Note: you should have set the values in Shopify settings before. Only then will the values show up in your store.

To get more information about adding countries and regions to your Shopify store, please read the Managing Markets article.