When a RadConfirm dialog appears on deleting or another user action is performed a dialog is opened with OK and Cancel button. The localization properties of these buttons can be altered via the RadEditor.Dialogs.resx, although modifying the valu eof the Common_OK property does not change the button's text. 

The following workaround changes the localization used by the RadWindowManager used by the FileExplorer and modifies the value as per to the desired one.

<script type="text/javascript">
	function OnClientLoad(sender, args) {
		var currentLocalization = sender.get_windowManager().get_localization();
		// Change the OK button localization value with "MyCustomOK"
		var myLocalization = currentLocalization.replace("\"OK\":\"OK\"", "\"OK\":\"MyCustomOK\"");

		sender.get_windowManager().set_localization(myLocalization);
	}
</script>

<telerik:RadFileExplorer runat="server" ID="RadFileExplorer1" OnClientLoad="OnClientLoad">
	<Configuration ViewPaths="~/" DeletePaths="~/" />
</telerik:RadFileExplorer>