Thank you for contacting us. If you want to change the button text, I'd suggest using the localization approach: http://www.telerik.com/help/windows-phone/localization-string-loaders.html
The strings you should be looking for are: OkButtonText and CancelButtonText.
In the meantime, we will see how to improve the existing mechanism. Let me know if the above works for you.
All the best,
the Telerik team
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