I've discovered that core/units/helpers/site_helper.php is handling default country parameter, and it's overriding in-commerce order_config unit.
All happens in line 31: $country = 'USA';
I propose either to put this as a config parameter from admin, or simply remove it completely, and display a blank country dropdown when user need to select a country.
Talking about this country dropdown, I must admit that I'm used -for years- to manually remove from DB unwanted countries, to have a shorter list. I suggest that we could display only available countries, as setup in admin shipping countries.
Additionally, we may inspect if default country declarations in order_config are still needed.