Let me explain in some more details. In general we provide localization support for controls that have static labels - such controls are RadScheduler
for example (you can click on the respective control and get to its respective demo, please note that when changing between the different languages only these static "labels" change). On the other side, controls like the RadMenu and the RadTabstrip for instance do not have such static labels that can change when setting a different language. Therefore my best suggestion would be to create different data sources and when switching between the languages you will also need to change the data source accordingly.
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 RadControls for ASP.NET AJAX, subscribe to their blog feed