code: public static HtmlTree HTML(String lang, Content head, Content body) {
          HtmlTree htmltree = new HtmlTree(HtmlTag.HTML, nullCheck(head), nullCheck(body));
          htmltree.addAttr(HtmlAttr.LANG, nullCheck(lang));
          return htmltree;
      }
docs: Generates an HTML tag with lang attribute. It also adds head and body
  content to the HTML tree.
  
  @param lang language for the HTML document
  @param head head for the HTML tag
  @param body body for the HTML tag
  @return an HtmlTree object for the HTML tag