By the new SpellChecker control I suppose you mean RadSpellChecker. It uses RadDictionary to keep the correct words for a specific language. Here you can find all dictionaries available in .tdf format and prepare RadDictionaries out of them as described in this article.
In you application you can have many dictionaries loaded but use only one at a time. Every dictionary is mapped to a culuture (CultureInfo) and thus whatever the current culture is, the appropriate dictionary is used. You can set the culture for the TextBoxSpellchecker as follows:
ControlSpellCheckerBase controlSpellChecker = (TextBoxSpellChecker)ControlSpellCheckersManager.GetControlSpellChecker(
You can read more about RadSpellChecker here
the Telerik team