Yet another example to add lang attribute to html tag, this time with Viritin add-on

MyUI.java

package org.test;

import javax.servlet.annotation.WebServlet;

import com.vaadin.annotations.Theme;
import com.vaadin.annotations.VaadinServletConfiguration;
import com.vaadin.server.VaadinRequest;
import com.vaadin.server.VaadinServlet;
import com.vaadin.ui.Label;
import com.vaadin.ui.UI;
import org.vaadin.viritin.util.HtmlElementPropertySetter;

/**
 * A UI example to set lang attr to document element
 */
@Theme("mytheme")
public class MyUI extends UI {

    @Override
    protected void init(VaadinRequest vaadinRequest) {
        // HtmlElementPropertySetter is part of Viritin add-on https://vaadin.com/directory#!addon/viritin
        HtmlElementPropertySetter heps = new HtmlElementPropertySetter(this);
        heps.setProperty("//html", "lang", "fr");

        setContent(new Label("Hello lang attr"));
    }

    @WebServlet(urlPatterns = "/*", name = "MyUIServlet", asyncSupported = true)
    @VaadinServletConfiguration(ui = MyUI.class, productionMode = false)
    public static class MyUIServlet extends VaadinServlet {

    }
}