setLanguageName example

    else {
      // Template required.       $filename = 'drupal.pot';
    }

    $item = $reader->readItem();
    if (!empty($item)) {
      $uri = $this->fileSystem->tempnam('temporary://', 'po_');
      $header = $reader->getHeader();
      $header->setProjectName($this->config('system.site')->get('name'));
      $header->setLanguageName($language_name);

      $writer = new PoStreamWriter();
      $writer->setURI($uri);
      $writer->setHeader($header);

      $writer->open();
      $writer->writeItem($item);
      $writer->writeItems($reader);
      $writer->close();

      $response = new BinaryFileResponse($uri);