public MainPage(){
            InitializeComponent();
            if (phoneAppService.State.TryGetValue("currentLanguage", out someObject))
            {  // Yes: go on
                var uri = "/Pages/Page_A.xaml";
                this.Dispatcher.BeginInvoke(() => this.NavigationService.Navigate(new Uri(uri, UriKind.Relative)));
            }
            else
            {  // No: select language before proceeding
                var uri = "/Pages/Page_B.xaml";
                this.Dispatcher.BeginInvoke( () => this.NavigationService.Navigate(new Uri(uri, UriKind.Relative)));
            }
}

    **// if previous page was Page_A or Page_B then exit application**
    protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
    {
       string sourcePage = "";
       if (NavigationContext.QueryString.TryGetValue("from", out sourcePage)) {
            if ((string.Compare(sourcePage.ToString(), "Page_A")) == 0 ? true : false) {
                **// EXIT APPLICATION**
            }
            if ((string.Compare(sourcePage.ToString(), "Page_B")) == 0 ? true : false) {
                **// EXIT APPLICATION**
            }
       }
        base.OnNavigatedTo(e);
    }

// Back Button pressed: notify MainPage so it can exit application
protected override void OnBackKeyPress(System.ComponentModel.CancelEventArgs e)
{
NavigationService.Navigate(new Uri(uri, UriKind.Relative));
base.OnBackKeyPress(e);
}

// Back Button pressed: notify MainPage so it can exit application
  protected override void OnBackKeyPress(System.ComponentModel.CancelEventArgs e)
        {
            var uri = "/MainPage.xaml?from=Page_B";
            NavigationService.Navigate(new Uri(uri, UriKind.Relative));
            base.OnBackKeyPress(e);
        }

RootFrame.Navigated += RootFrame_Navigated;

void RootFrame_Navigated(object sender, NavigationEventArgs e)
{
    var pageBURI = "/Pages/Page_B.xaml";
    var pageAURI = "/Pages/Page_A.xaml";

    if ((e.Uri == pageAURI || e.Uri == pageBURI) && RootFrame.BackStack.Count() > 0)
    {
        RootFrame.RemoveBackEntry();
    }
}