The spell checking in RadRichTextBox is enabled by default. You have control over it through the property IsSpellCheckingEnabled.
The dictionary that it uses is located in Telerik.Windows.Documents.Proofing.Dictionaries.En-US and you should add a reference to the assembly in order to load the dictionary. It is usually discovered through MEF, however, if you are using library caching, MEF will not be able to load the assembly, but you can proceed as described in this forum thread.
If that is not the case and you do not manage to get spell checking working properly, we would greatly appreciate some more information on your scenario and implementation. Preferably, a demo project.
All the best,
the Telerik team
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