NaF has beneficial properties for teeth. However, adding it to the water supply has been shown in several studies to be a complete waste of time. (ie...no statistical data correlation of increases or decreases in decay among children over 5 years old).
So, it has no benefit at all and increases risk of poisoning the water supply (as per box's post).
As to the rest of your post...not in the mood to try and translate it to english nor try and find the relationship between the 2 points.