function switchLanguageForm() {
      $fields = new FieldSet(new OptionsetField($name="chooseLang",
$title="Sprache wählen",
$source = array('de_DE' => 'Deutsch', 'en_GB' => 'English')));   
      $actions = new FieldSet(new FormAction('doSwitchLanguage', 'Sprache wechseln'));
      return new Form($this, 'switchLanguageForm', $fields, $actions);
   }

function doSwitchLanguage($data, $form) {
//Here i want to know which language was selected
//How can this be done?
}