As of v8.3.
An administrator can add country names with their ISO 3166-1 alpha-2 codes (at Administration > Address Countries). Added countries will be available in autocompletion in Address fields. Countries marked as preferred will be displayed first (on empty input).
Countries can also be imported from a CSV file.
The default predefined country list with English names can be optionally loaded. To do this, follow: Administration > Address Countries > top-right menu > Populate with default country list.