/**
* Change the default country on the checkout for non-existing users only
*/
add_filter( 'default_checkout_billing_country', 'change_default_checkout_country', 10, 1 );

function change_default_checkout_country( $country ) {
// If the user already exists, don't override country
if ( WC()->customer->get_is_paying_customer() ) {
return $country;
}

return 'NL'; // Override default to Netherlands (an example)
}