Many countries around the world have natural fluoride in their groundwater (much of Europe), use other dietary sources for fluoride (salt), or have a significant dietary component that naturally contains fluoride (tea).
What makes water fluoridation good public health policy isn’t that we do it blindly; it’s that it gets balanced against demographic and geographic factors that don’t necessary apply everywhere else in the world.
That map shows regions exceeding 1.5mg/L, i.e. over twice the amount that the US and other countries place in their municipal water supplies. It’s not a map of places with suitable concentrations of fluoride in groundwater; it shows excess amounts.
To my understanding, both Switzerland and Germany have around 1.0mg/L fluoride in much of their groundwater. This puts them firmly above municipal levels in the US, but below what your map shows.
What makes water fluoridation good public health policy isn’t that we do it blindly; it’s that it gets balanced against demographic and geographic factors that don’t necessary apply everywhere else in the world.