code: def ValidateLanguageCode(lang, column_name=None, problems=None):
    
    if IsEmpty(lang) or IsValidLanguageCode(lang):
      return True
    else:
      if problems:
        problems.InvalidValue(column_name, lang)
      return False
docs: Validates a non-required language code value using IsValidLanguageCode():
      - if invalid adds InvalidValue error (if problems accumulator is provided)
      - an empty language code is regarded as valid! Otherwise we might end up
        with many duplicate errors because of the required field checks.