Return the collection that represents a specific language or task.

    Args:
      lang (string): Language code.
      task (string): Task name.

def get_collection(self, lang=None, task=None):
    """ Return the collection that represents a specific language or task.

    Args:
      lang (string): Language code.
      task (string): Task name.
    """
    if lang: id = "{}{}".format(Downloader.LANG_PREFIX, lang)
    elif task: id = "{}{}".format(Downloader.TASK_PREFIX, task)
    else: raise ValueError("You should pass either the task or the lang")
    try:
      return self.info(id)
    except ValueError as e:
      if lang: raise LanguageNotSupported("Language {} is not supported".format(id))
      if task: raise TaskNotSupported("Task {} is not supported".format(id))