The RadSlider has three tooltips that may require localization - on the increase/decrease buttons and on the drag handle. The strings that will be displayed can be controlled with the following properties:
DecreaseText - the tooltip for the decrease button
IncreaseText - the tooltip for the increase button
DragText - the tooltip for the drag handle
These properties can be set to the desired string directly in the markup of the control, in the code-behind or via global resources in the markup to match the rest of the site's localization.