code: func (r Resources) MergeByLanguageInterface(in interface{}) (interface{}, error) {
  	r2, ok := in.(Resources)
  	if !ok {
  		return nil, fmt.Errorf("%T cannot be merged by language", in)
  	}
  	return r.MergeByLanguage(r2), nil
  }
docs: // MergeByLanguageInterface is the generic version of MergeByLanguage. It
  // is here just so it can be called from the tpl package.