code: public Subject addSubject(String language, String subject) {
          language = determineLanguage(language);
          Subject messageSubject = new Subject(language, subject);
          subjects.add(messageSubject);
          return messageSubject;
      }
docs: Adds a subject with a corresponding language.
  
  @param language the language of the subject being added.
  @param subject the subject being added to the message.
  @return the new {@link org.jivesoftware.smack.packet.Message.Subject}
  @throws NullPointerException if the subject is null, a null pointer exception is thrown