Filter taxonomy terms by term_order

/**
 * Filter taxonomy terms by term_order if term_order is specified
 *
 * @param string $orderby Method to order terms by
 * @param array  $query_vars The query
 * @param array  $taxonomies Array of taxonomies
 */
function filter_terms_order( $orderby, $query_vars, $taxonomies ) {
	return 'term_order' === $query_vars['orderby'] ? 'term_order' : $orderby;
}

add_filter( 'get_terms_orderby', 'filter_terms_order', 10, 3 );