Add a custom global product tab in WooCommerce

add_filter( 'woocommerce_product_tabs', 'woo_new_product_tab' );
function woo_new_product_tab( $tabs ) {
	
	// Adds the new tab
	
	$tabs['test_tab'] = array(
		'title' 	=> __( 'New Product Tab', 'woocommerce' ),
		'priority' 	=> 50,
		'callback' 	=> 'woo_new_product_tab_content'
	);

	return $tabs;

}
function woo_new_product_tab_content() {

	// The new tab content

	echo '<h2>New Product Tab</h2>';
	echo '<p>Here\'s your new product tab.</p>';
	
}

Replace the description tab with a custom function in WooCommerce

add_filter( 'woocommerce_product_tabs', 'woo_custom_description_tab', 98 );
function woo_custom_description_tab( $tabs ) {

	$tabs['description']['callback'] = 'woo_custom_description_tab_content';	// Custom description callback

	return $tabs;
}

function woo_custom_description_tab_content() {
	echo '<h2>Custom Description</h2>';
	echo '<p>Here\'s a custom description</p>';
}

Rename product tabs in WooCommerce

add_filter( 'woocommerce_product_tabs', 'woo_rename_tabs', 98 );
function woo_rename_tabs( $tabs ) {

	$tabs['description']['title'] = __( 'More Information' );		// Rename the description tab
	$tabs['reviews']['title'] = __( 'Ratings' );				// Rename the reviews tab
	$tabs['additional_information']['title'] = __( 'Product Data' );	// Rename the additional information tab

	return $tabs;

}

Change the default state and country on the checkout in WooCommerce

/** 
 * Manipulate default state and countries
 * 
 * As always, code goes in your theme functions.php file
 */
add_filter( 'default_checkout_country', 'change_default_checkout_country' );
add_filter( 'default_checkout_state', 'change_default_checkout_state' );
 
function change_default_checkout_country() {
  return 'XX'; // country code
}
 
function change_default_checkout_state() {
  return 'XX'; // state code
}