add_filter('post_link', 'area_permalink', 10, 3);
add_filter('post_type_link', 'area_permalink', 10, 3);

function area_permalink($permalink, $post_id, $leavename) {
    if (strpos($permalink, '%area%') === FALSE) return $permalink;

        // Get post
        $post = get_post($post_id);
        if (!$post) return $permalink;

        // Get taxonomy terms
        $terms = wp_get_object_terms($post->ID, 'area');
        if (!is_wp_error($terms) && !empty($terms) && is_object($terms[0])) $taxonomy_slug = $terms[0]->slug;
        else $taxonomy_slug = 'nation';

    return str_replace('%area%', $taxonomy_slug, $permalink);