As of Q1 SP1 version of RadSpellChecker, you can use built-in localization mechanism to change the text of the button. Unfortunately, however there is no way to replace the message box with a popup.
You can read more about localizing Telerik controls here. Please find attached a resource file with all the resource keys used in RadSpellChecker localization. The one you are interested in is Documents_SpellCheckingDialog_Change.
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