Hello Simon Chan,
Thank you for your interest in the RadControls for Silverlight.
control is a wrapper around an html frame. This way the Silverlight code (i.e. the Localization Manager) is separated from the content of the HTMLPlaceHolder. Regarding this deep separation we still not implement a mechanism for localizing the html content.
But as we always try to satisfy our clients, could you share with us what are your goals and what is the role of the Localization Manager in your scenario.
the Telerik team