How to add a country selector?

Go Online Store - Customize - Header section. Here you can see the Show country selector field. Tick a checkbox to show it in the header. As a result, you will see the list of countries. You can also choose the position of the selector between the Header and Top Bar. 

Please note that you should have set the values in Shopify settings before. And then the selector will show up in your store.

To set up the country in Shopify configuration, see the instructions in the article.

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