RadSpell could be easily localized by switching the Language property. This will affect the dialog labels, buttons, and error messages, not the spellchecking language.
The default installation of RadSpell comes with localization files for English, French and German languages. If you want to localize it for your specific language (Hebrew for example) you need to create new localization file. You can also check the Localization section of the Code Library to see if someone has already posted the language pack you are looking for.
Since localization is done in the standard ASP.NET way with Global Resources you need to copy the .resx files for RadSpell to your App_GlobalResources folder. You can find the built-in language packs in your Telerik UI installation folder (usually C:\Program Files [(x86) if on a 64bit Windows]\Telerik\UI for ASP.NET AJAX [QX YYYY]\App_GlobalResources ). You need to copy the RadSpell.Dialog.resx file along with the language specific files you are going to use (for example - RadSpell.Dialog.de-DE.resx).