While I was learning WPF I saw that there was a spelling check possibility added to the textbox and richttextbox. It works nicely, the only thing that I didn't found out then was how to change the language library so that you could use the spelling check for the Dutch language instead of the English (and default) one.

Finally I have more info about it:

First of all at this moment only the English, French and German language library's are available (I quote that this was at the end of February 2007).

When having a textbox, the language of the spellchecker depends on the language of the control, while for the richtextbox this depends on the typed text.

For the textbox you could change your language using this statement: x:lang="en-us" or x:lang="fr-fr".

You can find more info here: http://msdn2.microsoft.com/en-us/library/ms788718.aspx

Yesterday I also saw a blog article by Gill Cleeren, where he stated that the spell check comes from the Office dictionary. Have to look at it.