code: private void render(StringBuilder buf, int key, String undef, boolean expanded) {
      boolean force = key != VARIANT && (key == LANGUAGE || expanded);
      String value = fields[key];
      if (value != null || force) {
        if (buf.length() > 0) {
          buf.append(SEP);
        }
        buf.append(value == null ? undef : value);
      }
    }
docs: Render a locale field to the buffer. Language field is always emitted,
  and variant field is always omitted unless it has a value.