As I see you cannot find the error, so add after $db = Factory::getDbo(); in helper.php file the following code.
$languageTag = JFactory::getLanguage()->getTag();
$languageFilter = $db->Quote($languageTag).", ".$db->Quote('*');
$query = $db->getQuery(true);
$query->select('a.*, a.id AS spsimpleportfolio_item_id , a.tagids AS spsimpleportfolio_tag_id, a.created AS created_on')
->from($db->quoteName('#__spsimpleportfolio_items', 'a'))
->where($db->quoteName('a.published') . ' = 1');
if ($languageFilter) {
$query->where($db->quoteName('a.language') . ' IN (' . $languageFilter . ')');
}