When the Label property for RadTextBox is set it is rendered as a <label> element on the page. The text that is entered in it is not automatically translated.
If you would like to have different labels you would need to set them dynamically. Additionally you could localize the controls on a page by setting the Culture
Moreover our controls support localization through global resources. Take a look at this article
that describes the process for RadCalendar. Also different localizations for some or our controls are available here
If you have additional queries do not hesitate to contact us again.