There is a way to plug-in custom SpellCheckers by implementing the ISpellChecker interface and setting it to the SpellChecker property of RadRichTextBox. This point is included in our online documentation.
However, when it comes to scenarios such as using the Google spell checking API, we have not considered such extensions at this point, because it will take the spell-checking to the server, as opposed to the current client-side implementation. This will most probably cause performance issues due to the fact that requests will have to be transferred via WCF RIA services for each word in the document and will add little value to the feature.
I hope this answers your questions.
the Telerik team
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