The list with suggestions is taken from the TDF dictionary that RadSpell is configured to use. By default, RadSpell uses the en-US.tdf dictionary. If you want to use your own dictionary, you should put this TDF file in the App_Data/RadSpell folder and set the DictionaryLanguage property:
<telerik:RadSpell ID="RadSpell1" runat="server" DictionaryLanguage="Dictionary_Name"
You can find more information in the following article: http://www.telerik.com/help/aspnet-ajax/spell_dictionariesoverview.html
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