Answer: When accessing your Theme Editor, select your Header section or your Footer bottom section, then check the Enable country/region selector box.
For your country/region selector to function, your markets must be set up on your store:
Step 1: Within your Shopify admin, go to under Settings.
Step 2: Select Markets and add the countries/regions you want to sell to.
For more information, see Shopify's help pages on Markets.