$lang = JFactory::getLanguage();
$db = JFactory::getDBO();
$query = 'SELECT sitename FROM #__languages WHERE lang_code = "' . $lang->getTag() . '"';
$db->setQuery($query);
$sitename = $db->loadResult();
// If no sitename defined - show default sitename
if (!$sitename){
$app = JFactory::getApplication();
$sitename = $app->getCfg('sitename');
}