code: public void setLanguage(String lang, boolean preferred) {
          PdfDictionary usage = getUsage();
          PdfDictionary dic = new PdfDictionary();
          dic.put(PdfName.LANG, new PdfString(lang, PdfObject.TEXT_UNICODE));
          if (preferred)
              dic.put(PdfName.PREFERRED, PdfName.ON);
          usage.put(PdfName.LANGUAGE, dic);
      }
docs: Specifies the language of the content controlled by this
  optional content group
  @param lang a language string which specifies a language and possibly a locale
  (for example, <B>es-MX</B> represents Mexican Spanish)
  @param preferred used by viewer applications when there is a partial match but no exact
  match between the system language and the language strings in all usage dictionaries