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How to check if a variable has already been declared or not? (IsolatedStorage)
RootFrame.Navigating += new NavigatingCancelEventHandler(MainPage.RootFrame_Navigating);

public static void RootFrame_Navigating(object sender, NavigatingCancelEventArgs e)
{
    if (e.Uri.ToString().Contains("/MainPage.xaml") != true)
        return;
    // Change the current navigation
    e.Cancel = true;
    App.RootFrame.Dispatcher.BeginInvoke(delegate
    {
        string confronto = (string)MainPage.Impostazioni.appSettings["lingua"];
        if ((confronto == "italiano") || (confronto == "english"))
            App.RootFrame.Navigate(new Uri("/MainPage.xaml?method=cancel%20navigation&time=", UriKind.Relative));
        else
            App.RootFrame.Navigate(new Uri("/StartMenu.xaml?method=cancel%20navigation&time=", UriKind.Relative));
    });
}

private void engLink_Click(object sender, RoutedEventArgs e)
{
    Impostazioni.appSettings.Add("language", "english");
}

private void itaLink_Click(object sender, RoutedEventArgs e)
{
    Impostazioni.appSettings.Add("language", "italiano");
}

If (IsolatedStorageSettings.ApplicationSettings.Contains("language"))
{
    ...
}

Impostazioni.ApplicationSettings.Add("language","");

private void engLink_Click(object sender, RoutedEventArgs e)
    {
        Impostazioni.appSettings.Add("language", "english");
    }