The comma comes from your regional settings.
If they were e.g. English you would see a point.
The article below shows how you can modify the auto tool tip.
http://joshsmithonwpf.wordpress.com/2007/09/14/modifying-the-auto-tooltip-of-a-slider/[
^]
Another way could be to suppress the auto tool tip
and show a text block below the slider which is data bound to the slider.
Then you could use any value converter you like.