Get all Polylang posts that don't have a language set.

polylang-posts-without-language.php

<?php

new \WP_Query([
    'post_type'      => ['post', 'page'],
    'posts_per_page' => -1,
    'lang'           => '',
    'fields'         => 'ids',
    'tax_query' => [
        [
            'taxonomy'  => 'language',
            'operator'  => 'NOT EXISTS',
        ],
    ],
]);