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.
Regards,
Rumen
Telerik
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