Remember to the set the locale

Per Component
Session Default

This includes things like names of months, 12 or 24 hour time formats, the order of days and months in dates, and decimal separators in numbers.

Displaying values in a locale foreign to the user is bad enough. Forcing your users to enter values in a foreign locale (such as decimal separator periods instead of commas, or vice versa) can be considered a cruel (although sadly not particularly unusual) punishment.

A Vaadin application’s locale, if not explicitly set, defaults to the locale defined in the request headers sent by the browser , or, if that’s missing, the locale returned by java.util.Locale.getDefault(), which in turn is the default locale of the Java Virtual Machine, which depends on the configuration of the environment in which it happens to be running. Either way, chances are that the end result does not match what your users expect.

Per Component

Vaadin components have a setLocale() method that allows specifying the locale separately for each component.

InlineDateField datePicker = new InlineDateField();
datePicker.setLocale( java.util.Locale.CANADA_FRENCH );

Inheritance

A component inherits its locale from its parent. So setting the locale on a component affects its nested children components by default. Remember that layouts and even the UI (Vaadin 7) object are components too, so setting their locale affects their content.

Session Default

Instead of setting a locale for each individual component, you may set a Session-wide default. The components inherit that property. You may continue to override this session-default locale for individual components, where needed, with a call to setLocale.

Setting the default locale changed between Vaadin 6 and 7.

In Vaadin 6

In Vaadin 6, you set the locale by calling setLocalemethod on the Application object. See chapter 12.14 Accessing Session-Global Data of the Book of Vaadin, Vaadin 6 edition, for important information about accessing the Application object.

In Vaadin 7

In Vaadin session, you must go through the current VaadinSession object to call setLocale. You may access that object in either of two ways:

Through that class’ static method getCurrent
Jumping from a component object to a UI object to the current VaadinSession object.

Bug & Workaround\\

Unfortunately, there is a bug (or design issue) in Vaadin 7. See ticket 12350. The problem is that setting the locale of the VaadinSession does not affect already existing UI objects. So, while the init method of your app’s UI is the natural place to set the session’s locale, doing so does not take effect in that current UI until after a page refresh.

The workaround is to explicitly set the current UI object’s locale, in addition to setting the VaadinSession object’s locale. The first affects the current UI, while the second affects any future UI objects that may be instantiated in your app.

This ( if in a UI object’s "init" method)…

Locale locale = Locale.CANADA_FRENCH;
this.setLocale( locale ); // Call to affect this current UI. Workaround for bug: http://dev.vaadin.com/ticket/12350
this.getSession().setLocale( locale ); // Affects only future UI instances, not current one because of bug. See workaround in line above.
// VaadinSession.getCurrent().setLocale( locale ); // Alternative to "this.getSession".

…or this ( if not in a UI object, add "getUI()" )…

Locale locale = Locale.CANADA_FRENCH;
this.getUI().setLocale( locale ); // Call to affect this current UI. Workaround for bug: http://dev.vaadin.com/ticket/12350
this.getUI().getSession().setLocale( locale ); // Affects only future UI instances, not current one because of bug. See workaround in line above.
// VaadinSession.getCurrent().setLocale( locale ); // Alternative to "this.getSession".

Example App

Here is source code for a complete Vaadin 7 example app.

package com.example.vaadinexperiment;

import com.vaadin.annotations.Theme;
import com.vaadin.annotations.VaadinServletConfiguration;
import com.vaadin.server.VaadinRequest;
import com.vaadin.server.VaadinServlet;
import com.vaadin.server.VaadinSession;
import com.vaadin.ui.InlineDateField;
import com.vaadin.ui.UI;
import com.vaadin.ui.VerticalLayout;
import java.util.Locale;
import javax.servlet.annotation.WebServlet;

@Theme ( "mytheme" )
@SuppressWarnings ( "serial" )
public class MyVaadinUI extends UI
{

    @WebServlet ( value = "/*" , asyncSupported = true )
    @VaadinServletConfiguration ( productionMode = false , ui = MyVaadinUI.class , widgetset = "com.example.vaadinexperiment.AppWidgetSet" )
    public static class Servlet extends VaadinServlet
    {
    }

    @Override
    protected void init ( VaadinRequest request )
    {
        final VerticalLayout layout = new VerticalLayout();
        layout.setMargin( true );
        layout.setSpacing( true );
        setContent( layout );

        Locale locale = Locale.CANADA_FRENCH;
        this.setLocale( locale ); // Call to affect this current UI. Workaround for bug: http://dev.vaadin.com/ticket/12350
        this.getSession().setLocale( locale ); // Affects only future UI instances, not current one because of bug. See workaround in line above.
        // VaadinSession.getCurrent().setLocale( locale ); // Alternative to "this.getSession".

        InlineDateField datePicker = new InlineDateField();
        datePicker.setCaption( "This component’s locale → defaults to parent’s, → which defaults to VaadinSession’s, → which defaults to JVM" );
        layout.addComponent( datePicker );

        InlineDateField datePickerFinnish = new InlineDateField();
        datePickerFinnish.setCaption( "This component’s default locale is overridden for Finnish" );
        datePickerFinnish.setLocale( new Locale( "fi" , "FI" ) );
        layout.addComponent( datePickerFinnish );
    }

}

Calendar

Information here drawn from this StackOverflow.com question.

Docs