Example how to add lang attribute to Vaadin app

MyUI.java

package org.test;

import javax.servlet.annotation.WebServlet;

import com.vaadin.annotations.Theme;
import com.vaadin.annotations.VaadinServletConfiguration;
import com.vaadin.server.BootstrapFragmentResponse;
import com.vaadin.server.BootstrapListener;
import com.vaadin.server.BootstrapPageResponse;
import com.vaadin.server.VaadinRequest;
import com.vaadin.server.VaadinServlet;
import com.vaadin.ui.Label;
import com.vaadin.ui.UI;
import javax.servlet.ServletException;

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

    @Override
    protected void init(VaadinRequest vaadinRequest) {
        setContent(new Label("Hello lang attr"));
    }

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

        @Override
        protected void servletInitialized() throws ServletException {
            super.servletInitialized();
            getService().addSessionInitListener(e ->
                e.getSession().addBootstrapListener(new BootstrapListener() {
                    @Override
                    public void modifyBootstrapFragment(
                            BootstrapFragmentResponse response) {
                        // NOP, this is for portlets etc
                    }

                    @Override
                    public void modifyBootstrapPage(
                            BootstrapPageResponse response) {
                        response.getDocument().child(0).attr("lang", "fr");
                    }
                })
            );
        }

    }
}