Dictionary for De-at.xaml

    <ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                        xmlns:s="clr-namespace:System;assembly=mscorlib"
                        xmlns:local="clr-namespace:MutipleLanguages">
        <s:String x:Key="btnswitch"> , </s:String>
        <s:String x:Key="buttonNewTaskWindow">Neu mission</s:String>
        <s:String x:Key="buttonProperty">Mission eigenschaften</s:String>
    </ResourceDictionary>
    

Dictionary for En-us.xaml

    <ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                        xmlns:s="clr-namespace:System;assembly=mscorlib"
                        xmlns:local="clr-namespace:MutipleLanguages">
        <s:String x:Key="btnswitch">Switch</s:String>
        <s:String x:Key="buttonNewTaskWindow">New Task</s:String>
        <s:String x:Key="buttonProperty">Task Property</s:String>
    </ResourceDictionary>

In App.xaml
    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="Resource\En-us.xaml"/>
                <ResourceDictionary Source="Resource\De-at.xaml"/>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>
</Application>

In the MianWindow.xaml
<StackPanel>
        <Button Width="100"  Margin="10" x:Name="btnswitch" Content="{DynamicResource btnswitch}" Click="btnswitch_Click"/>
        <Button Width="100"  Margin="10" x:Name="buttonNewTaskWindow" Content="{DynamicResource buttonNewTaskWindow}"/>
        <Button Width="100"  Margin="10"  x:Name="buttonProperty" Content="{DynamicResource buttonProperty}"/>
</StackPanel>

MianWindow.xaml.cs

  private void btnswitch_Click(object sender, RoutedEventArgs e)
        {
            List<ResourceDictionary> dictionaryList = new List<ResourceDictionary>();
            foreach (ResourceDictionary dictionary in Application.Current.Resources.MergedDictionaries)
            {
                dictionaryList.Add(dictionary);
            }
            string strBtnSwitch = this.btnswitch.Content.ToString();
            if(strBtnSwitch == "Switch")
            {
                string requestedCulture = @"Resource\De-at.xaml";
                ResourceDictionary resourceDictionary = dictionaryList.FirstOrDefault(d => d.Source.OriginalString.Equals(requestedCulture));
                Application.Current.Resources.MergedDictionaries.Remove(resourceDictionary);
                Application.Current.Resources.MergedDictionaries.Add(resourceDictionary);
            }
            else
            {
                string requestedCulture = @"Resource\En-us.xaml";
                ResourceDictionary resourceDictionary = dictionaryList.FirstOrDefault(d => d.Source.OriginalString.Equals(requestedCulture));
                Application.Current.Resources.MergedDictionaries.Remove(resourceDictionary);
                Application.Current.Resources.MergedDictionaries.Add(resourceDictionary);
            }            
        }