SpellCheck.IsEnabled="True" in .NET 4.0 (not working with german .NET 4.0 Framework)

I have an WPF App, where I enabled the Spell-Checker for some TextBoxes. When I run it locally it works well, but not when I run it at my clients site, on Win.开发者_如何学运维 XP PCs with .NET 4.0 (German) installed.

Actually It should access the German language dictionary, so I really not get the point.

Somehow I must have missed something. Can you help me?

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The actual solution was to load the corresponding language pack extra from the microsoft website and install it.
I got wrong information from my client, language packs were not installed additionaly yet.