code: public boolean containsLanguage (@Nullable final String sLanguage)
    {
      if (sLanguage == null)
        return false;
  
      final String sValidLanguage = LocaleHelper.getValidLanguageCode (sLanguage);
      if (sValidLanguage == null)
        return false;
      return m_aRWLock.readLocked ( () -> m_aLanguages.contains (sValidLanguage));
    }
docs: Check if the passed language is known.
  
  @param sLanguage
  The language to check. May be <code>null</code>.
  @return <code>true</code> if the passed language is contained,
  <code>false</code> otherwise.