Here is the directory of my demo:
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
In the App.xaml
In the MianWindow.xaml
MianWindow.xaml.cs
Here is the code for you to use.