1. Открываем свойства проекта.
2. Идём во вкладку Application.
3. Открываем Assembly Information.
4. Выбираем нейтральную культуру
   
5. Жмём OK.

<Application.Resources>
	<ResourceDictionary>
		<ResourceDictionary.MergedDictionaries>
			<ResourceDictionary Source="Resources/lang.xaml" />
		</ResourceDictionary.MergedDictionaries>
	</ResourceDictionary>
</Application.Resources>

<v:String x:Key="m_Title">WPF Localization example</v:String>
<v:String x:Key="m_lblHelloWorld">Hello world!</v:String>
<v:String x:Key="m_menu_Language">Language</v:String>
<v:Double x:Key="m_Number">20.15</v:Double>

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
					xmlns:v="clr-namespace:System;assembly=mscorlib">
    <!-- Main window -->
    <v:String x:Key="m_Title">Пример WPF локализации</v:String>
    <v:String x:Key="m_lblHelloWorld">Привет мир!</v:String>
    <v:String x:Key="m_menu_Language">Язык</v:String>
    <v:Double x:Key="m_Number">10.5</v:Double>
</ResourceDictionary>

Class Application
    Private Shared m_Languages As New List(Of CultureInfo)

    Public Shared ReadOnly Property Languages As List(Of CultureInfo)
        Get
            Return m_Languages
        End Get
    End Property

    Public Sub New()
        m_Languages.Clear()
        m_Languages.Add(New CultureInfo("en-US")) 'Нейтральная культура для этого проекта
        m_Languages.Add(New CultureInfo("ru-RU"))
    End Sub
End Class

public partial class App : Application
{
   private static List<CultureInfo> m_Languages = new List<CultureInfo>();

	public static List<CultureInfo> Languages 
	{
		get 
		{
			return m_Languages;
		}
	}

	public App()
	{
		m_Languages.Clear();
		m_Languages.Add(new CultureInfo("en-US")); //Нейтральная культура для этого проекта
		m_Languages.Add(new CultureInfo("ru-RU"));
	}
}

'Евент для оповещения всех окон приложения
Public Shared Event LanguageChanged(sender As Object, e As EventArgs)

Public Shared Property Language As CultureInfo
	Get
		Return System.Threading.Thread.CurrentThread.CurrentUICulture
	End Get
	Set(value As CultureInfo)
		If value Is Nothing Then Throw New ArgumentNullException("value")
		If value.Equals(System.Threading.Thread.CurrentThread.CurrentUICulture) Then Exit Property

		'1. Меняем язык приложения:
		System.Threading.Thread.CurrentThread.CurrentUICulture = value

		'2. Создаём ResourceDictionary для новой культуры
		Dim dict As New ResourceDictionary()
		Select Case value.Name
			Case "ru-RU"
				dict.Source = New Uri(String.Format("Resources/lang.{0}.xaml", value.Name), UriKind.Relative)
			Case Else
				dict.Source = New Uri("Resources/lang.xaml", UriKind.Relative)
		End Select

		'3. Находим старую ResourceDictionary и удаляем его и добавляем новую ResourceDictionary
		Dim oldDict As ResourceDictionary = (From d In My.Application.Resources.MergedDictionaries _
											 Where d.Source IsNot Nothing _
											 AndAlso d.Source.OriginalString.StartsWith("Resources/lang.") _
											 Select d).First
		If oldDict IsNot Nothing Then
			Dim ind As Integer = My.Application.Resources.MergedDictionaries.IndexOf(oldDict)
			My.Application.Resources.MergedDictionaries.Remove(oldDict)
			My.Application.Resources.MergedDictionaries.Insert(ind, dict)
		Else
			My.Application.Resources.MergedDictionaries.Add(dict)
		End If

		'4. Вызываем евент для оповещения всех окон.
		RaiseEvent LanguageChanged(Application.Current, New EventArgs)
	End Set
End Property

//Евент для оповещения всех окон приложения
public static event EventHandler LanguageChanged;

public static CultureInfo Language {
	get 
	{
		return System.Threading.Thread.CurrentThread.CurrentUICulture; 
	}
	set
	{
		if(value==null) throw new ArgumentNullException("value");
		if(value==System.Threading.Thread.CurrentThread.CurrentUICulture) return;

		//1. Меняем язык приложения:
		System.Threading.Thread.CurrentThread.CurrentUICulture = value;

		//2. Создаём ResourceDictionary для новой культуры
		ResourceDictionary dict = new ResourceDictionary();
		switch(value.Name){
			case "ru-RU": 
				dict.Source = new Uri(String.Format("Resources/lang.{0}.xaml", value.Name), UriKind.Relative);
				break;
			default:
				dict.Source = new Uri("Resources/lang.xaml", UriKind.Relative);
				break;
		}

		//3. Находим старую ResourceDictionary и удаляем его и добавляем новую ResourceDictionary
		ResourceDictionary oldDict = (from d in Application.Current.Resources.MergedDictionaries
									  where d.Source != null && d.Source.OriginalString.StartsWith("Resources/lang.")
									  select d).First();
		if (oldDict != null)
		{
			int ind = Application.Current.Resources.MergedDictionaries.IndexOf(oldDict);
			Application.Current.Resources.MergedDictionaries.Remove(oldDict);
			Application.Current.Resources.MergedDictionaries.Insert(ind, dict);
		} 
		else
		{
			Application.Current.Resources.MergedDictionaries.Add(dict);
		}

		//4. Вызываем евент для оповещения всех окон.
		LanguageChanged(Application.Current, new EventArgs());
	}
}

Class MainWindow

    Public Sub New()
        InitializeComponent()

        'Добавляем обработчик события смены языка у приложения
        AddHandler Application.LanguageChanged, AddressOf LanguageChanged

        Dim currLang = Application.Language

        'Заполняем меню смены языка:
        menuLanguage.Items.Clear()
        For Each lang In Application.Languages
            Dim menuLang As New MenuItem()
            menuLang.Header = lang.DisplayName
            menuLang.Tag = lang
            menuLang.IsChecked = lang.Equals(currLang)
            AddHandler menuLang.Click, AddressOf ChangeLanguageClick
            menuLanguage.Items.Add(menuLang)
        Next
    End Sub

    Private Sub LanguageChanged(sender As Object, e As EventArgs)
        Dim currLang = Application.Language

        'Отмечаем нужный пункт смены языка как выбранный язык
        For Each i As MenuItem In menuLanguage.Items
            Dim ci As CultureInfo = TryCast(i.Tag, CultureInfo)
            i.IsChecked = ci IsNot Nothing AndAlso ci.Equals(currLang)
        Next
    End Sub

    Private Sub ChangeLanguageClick(sender As Object, e As RoutedEventArgs)
        Dim mi As MenuItem = TryCast(sender, MenuItem)
        If mi IsNot Nothing Then
            Dim lang As CultureInfo = TryCast(mi.Tag, CultureInfo)
            If lang IsNot Nothing Then
                Application.Language = lang
            End If
        End If
    End Sub

End Class

namespace WPFLocalizationCSharp
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();

            App.LanguageChanged += LanguageChanged;

            CultureInfo currLang = App.Language;

            //Заполняем меню смены языка:
            menuLanguage.Items.Clear();
            foreach (var lang in App.Languages)
            {
                MenuItem menuLang = new MenuItem();
                menuLang.Header = lang.DisplayName;
                menuLang.Tag = lang;
                menuLang.IsChecked = lang.Equals(currLang);
                menuLang.Click += ChangeLanguageClick;
                menuLanguage.Items.Add(menuLang);
            }
        }

        private void LanguageChanged(Object sender, EventArgs e)
        {
            CultureInfo currLang = App.Language;

            //Отмечаем нужный пункт смены языка как выбранный язык
            foreach (MenuItem i in menuLanguage.Items)
            {
                CultureInfo ci = i.Tag as CultureInfo;
                i.IsChecked = ci != null && ci.Equals(currLang);
            }
        }

        private void ChangeLanguageClick(Object sender, EventArgs e)
        {
            MenuItem mi = sender as MenuItem;
            if (mi != null)
            {
                CultureInfo lang = mi.Tag as CultureInfo;
                if (lang != null) {
                    App.Language = lang;
                }
            }

        }
    }
}

    Private Sub Application_LoadCompleted(sender As Object, e As NavigationEventArgs) Handles Me.LoadCompleted
        Language = My.Settings.DefaultLanguage
    End Sub
    Private Shared Sub OnLanguageChanged(sender As Object, e As EventArgs) Handles MyClass.LanguageChanged
        My.Settings.DefaultLanguage = Language
        My.Settings.Save()
    End Sub

private void Application_LoadCompleted(object sender, System.Windows.Navigation.NavigationEventArgs e)
{
	Language = WPFLocalizationCSharp.Properties.Settings.Default.DefaultLanguage;
}

private void App_LanguageChanged(Object sender, EventArgs e)
{
	WPFLocalizationCSharp.Properties.Settings.Default.DefaultLanguage = Language;
	WPFLocalizationCSharp.Properties.Settings.Default.Save();
}

LoadCompleted="Application_LoadCompleted"

App.LanguageChanged += App_LanguageChanged;