Languages that are covered by a specific task.

    Args:
      lang (string): Language code name.

def supported_tasks(self, lang=None):
    """Languages that are covered by a specific task.

    Args:
      lang (string): Language code name.
    """
    if lang:
      collection = self.get_collection(lang=lang)
      return [x.id.split('.')[0] for x in collection.packages]
    else:
      return [x.name.split()[0] for x in self.collections() if Downloader.TASK_PREFIX in x.id]