SearchWP + Polylang — force search results to include untranslated post types

functions.php

function searchwp_polylang_include_only_current_language_posts( $relevantPostIds, $engine, $terms ) {
  
  if ( isset( $engine ) ) {
    $engine = null;
  }

  if ( isset( $terms ) ) {
    $terms = null;
  }

  $post_ids = $relevantPostIds;

  if ( function_exists( 'pll_current_language' ) && function_exists( 'pll_default_language' ) ) {

    $currentLanguage = pll_current_language();

    if ( false == $currentLanguage ) {
      $currentLanguage = pll_default_language();
    }

    // get all posts in the current language
    $args_translated = array(
      'nopaging'      => true,
      'post_type'     => array('translated_post_type1', 'translated_post_type2'),
      'post_status'   => 'any',
      'fields'        => 'ids',
      'tax_query'     => array(
        array(
          'taxonomy'  => 'language',
          'field'     => 'slug',
          'terms'     => sanitize_text_field( $currentLanguage ),
        ),
      )
    );

    $query_translated = new WP_Query( $args_translated );
    $post_ids_translated = $query_translated->posts;


    $args_untranslated = array(
      'nopaging'  => true,
      'post_type' => array('untranslated_post_type1', 'untranslated_post_type2'),
      'post_status'   => 'any',
      'fields'    => 'ids',
    );
    $query_untranslated = new WP_Query( $args_untranslated );
    $post_ids_untranslated = $query_untranslated->posts;

    $post_ids = array_merge( $post_ids_untranslated, $post_ids_translated );
  }

  return $post_ids;
}

add_filter( 'searchwp_include', 'searchwp_polylang_include_only_current_language_posts', 10, 3 );