getPlurals example

    if (isset($langcode)) {
      $strings = $this->translateFilterLoadStrings();

      $plurals = $this->getNumberOfPlurals($langcode);

      foreach ($strings as $string) {
        // Cast into source string, will do for our purposes.         $source = new SourceString($string);
        // Split source to work with plural values.         $source_array = $source->getPlurals();
        $translation_array = $string->getPlurals();
        if (count($source_array) == 1) {
          // Add original string value and mark as non-plural.           $plural = FALSE;
          $form['strings'][$string->lid]['original'] = [
            '#type' => 'item',
            '#title' => $this->t('Source string (@language)', ['@language' => $this->t('Built-in English')]),
            '#title_display' => 'invisible',
            '#plain_text' => $source_array[0],
            '#prefix' => '<span lang="en">',
            '#suffix' => '</span>',
          ];