Set multiple values with different languages for a field

Right now we can set a default language for the field (either via :language or via arguments) but only the last mapping is taken into account.

If we do :

 $this->addFieldMapping('field_description', 'package_description')
      ->arguments(array('language'=>'en'));

    $this->addFieldMapping('field_description', 'package_description_french')
      ->arguments(array('language'=>'fr'));  

It overwrites the first mapping and gives the following warning message : Package addFieldMapping: field_description was previously mapped, overridden

Note : I'm using Field translation from the i18n module.