Thank you for your question.
First of all, we would like to mention that RadRichTextBox could use only one dictionary at a time for spell checking. You could find more information here.
Currently, RadRichTextBox doesn't allow spell checking in languages such as Persian, Hebrew and a few others, because they are interpreted as languages containing only upper case letters. Therefore, Persian text in your document is not marked with wavy line.
Fortunately, for the upcoming 2012 Q3 release expected next month we will add a property which will allow spell checking of words with only upper letters, which means that it could be used for spell checking Persian and similar language.
Another thing to mention is that our spell checking algorithm is developed mainly for left to right languages and may cause different behavior for right to left language. Our initial tests over it demonstrates satisfying results, so we are optimistic for the spell checking of the right to left languages.
I hope this information was helpful.
the Telerik team