I am glad that you solved the problem on your end,
By default the DictionaryPath property points to the ~/App_Data/RadSpell location. It is possible that the ~/App_Data/RadSpell folder resides on the local machine and for that reason the spellchecker is able to find its dictionaries. When deployed the precompiled site does not have a ~/App_Data/RadSpell folder and for this reason the spellchecker is unable to locate the TDF files. For that reason the developer should set the DictionaryPath property to point to the ~/bin/App_Data/RadSpell.
the Telerik team