If you use Language Selection (sr_language_menu) extension the default language selecting become impossible in some cases. It is caused by that code in class.tx_srlanguagemenu_pi1.php (repeated three times):

if(!$this->rlmp_language_detectionLoaded) $GLOBALS['TSFE']->linkVars = ereg_replace('&L=0' , '', $GLOBALS['TSFE']->linkVars);

So you have to delete these strings.