code: public Request acceptLanguage(String language){
          final String acceptLanguage = RequestHeaderFields.ACCEPT_LANGUAGE.getName();
          String languageValue = headers.get(acceptLanguage);
          if (languageValue == null){
              languageValue = language;
          } else {
              languageValue = languageValue + ", " + language;
          }
          return this.setHeader(acceptLanguage, languageValue);
      }
docs: sets the accept language header for the request, if exists then appends
  @param language   : header field value to add
  @return : Request Object with accept language header value set