This was solved by adding this code to the file components/com_droomla/views/droomla/view.html.php:
$item->introtext=iconv("UTF-8","ISO-8859-1", $item->introtext);after this line on line 554:
$item->introtext = $node->content . Drupal::convert_links( $node->links, 'droomla', $droomla_params->get( 'itemid', 1 ) );
This code forces encoding for the Czech character set.