Possible solution in export_responses (remotecontrol.php):

public function export_responses($sSessionKey, $iSurveyID, $sDocumentType, $sLanguageCode=null, $sCompletionStatus='all', $sHeadingType='full', $sResponseType='short', $iFromResponseID=null, $iToResponseID=null, $aFields=null)
{
....
$oFomattingOptions->headingFormat=$sHeadingType;

    // Check if $sResponseType is an array
    // if yes set the formatting Options like this.
    if (is_array($sResponseType)) {
      $oFomattingOptions->answerFormat=$sResponseType['answerFormat'];
      $oFomattingOptions->convertY=$sResponseType['convertY'];
      $oFomattingOptions->yValue=$sResponseType['yValue'];
      $oFomattingOptions->convertN=$sResponseType['convertN'];
      $oFomattingOptions->nValue=$sResponseType['nValue'];
    } 
    // if not, continue with the old behavior
    else {
      $oFomattingOptions->answerFormat=$sResponseType;
    }

    $oFomattingOptions->output='file';

....
return base64_encode($sFileData);
}

Call to the function would look like this:
$aFormat = array();
$aFormat['answerFormat']='short';
$aFormat['convertY']=true;
$aFormat['yValue']='1';
$aFormat['convertN']=true;
$aFormat['nValue']='2';

this->export_responses( $sessionKey, $survey_id,'csv','de','all','code',$aFormat);

or shorter:

export_responses( $sessionKey, $survey_id,'csv','de','all','code',array('answerFormat'=>'short', 'convertY'=>true, 'yValue'=>'1', 'convertN'=>true, 'nValue'=>'2'));