The following error messages within application/controllers/survey/index.php are not being translated although a translation in de.mo is available.

if ( $this->_isClientTokenDifferentFromSessionToken($clienttoken,$surveyid) )
{
$asMessage = array(
$clang->gT('Token mismatch'),
$clang->gT('The token you provided doesn\'t match the one in your session.'),
$clang->gT('Please wait to begin with a new session.')
);
$this->_createNewUserSessionAndRedirect($surveyid, $redata, LINE, $asMessage);
}

I think it true also for:

{
$asMessage = array(
$clang->gT('Previous session is set to be finished.'),
$clang->gT('Your browser reports that it was used previously to answer this survey. We are resetting the session so that you can start from the beginning.'),
$clang->gT('Please wait to begin with a new session.')
);
$this->_createNewUserSessionAndRedirect($surveyid, $redata, LINE, $asMessage);
}

My guess is, that the language to use is defined after the above error-message-code (sorry i'm not a programmer ;-) )