Pretty sure the Health & Safety at Work Act (UK) [1] meets this requirement. There's various common law things related to duty of care and negligence that probably apply less specifically too.
I'd really expect every democratic country to have some form of law requiring employers take care of their employees -- they should expect to do something in return for leeching off ones production of value.
That is correct, but the bar for enforcement is very high when it comes to mental health. E.g. I remember a case of a police officer suffering from depression (?) but "only" stressing folks out is not enough unfortunately.
In addition, you have to show a causal relationsship, which again is a lot more difficult for mental health as there are usually several factors at play even though work might be the pre-dominant one.
A small note: Germany does have a law like this. (I cannot find anything about it in English though.)
https://translate.google.com/translate?sl=auto&tl=en&u=https...