Please, make sure that you have set full ASPNET / NETWORK SERVICE account permissions to the dictionaries folder and to the es-ES dictionary.
For your convenience I prepared and attached a sample working project based on your code that works fine on my side. The spellchecker works as expected on my side and corrects the misspelled Spanish words.
In my test I used the latest Q3 SP2 version of RadControls for ASP.NET AJAX (Telerik.Web.UI.dll 2008.3 1314). Please, test the project and let me know if you still experience the problem.
the Telerik team
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