In the RadToolTip control, there is only one element which could be translated - its Close button, when you have set the ManualClose property to true. The localization mechanism is pretty complex and for the time being there are not a lot of customers' requests for embedding it in this control. That is why there is not such for now but we are considering implementing a corresponding property in the future.
What I can suggest as a workaround is to get the particular A element and set its title to the desired custom text. The mentioned element does not have an ID and that is why you can get a reference to it by using the getElementsByClassName function. It is not supported in IE and that is why you should also add its code on your page.
For your convenience I prepared a sample demo which shows how to achieve the desired behavior.