If you would like to change the default country in the final booking form field you can add below given code in your child theme functions.php file. To know how to create the child theme please refer to below given link: 

Child Theme  

After creating the child theme in your child theme functions.php file add below given code.

function wp_travel_customize_booking_form($fields) {
	$fields['traveller_fields']['country']['default'] = 'NL';
	$fields['billing_fields']['country']['default'] = 'NL';
 
	return $fields;
 }
 
 add_filter( 'wp_travel_checkout_fields', 'wp_travel_customize_booking_form', 99 );

You need to replace ‘NL‘ country code with your country code in above given code.