_getMessageForKeys(keys, locale) {
    if (typeof keys === 'string') {
      return this._getMessageForKey(keys, locale);
    }
    const reversedKeys = Array.from(keys).reverse(); // Array.from prevents mutation of argument
    let key;
    let message;
    while (reversedKeys.length) {
      key = reversedKeys.pop();
      message = this._getMessageForKey(key, locale);
      if (message) {
        return message;
      }
    }
    return undefined;
  }
@param {string | string[]} keys
@param {string} locale
@returns {string | undefined}
@protected