You can find the RadSpell's localization resx files in the App_GlobalResources
folder. It is located in the installation folder which is usually located at C:\Program Files [(x86) if on a 64bit Windows]\Telerik\UI for ASP.NET AJAX [QX YYYY]\App_GlobalResources.
The following help article provides more information: Localization
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the UI for ASP.NET AJAX, subscribe to the blog feed