To be eligible to vote, must a registrant demonstrate a certain minimum mental capacity, grasp of issues, ability to read/write the native language, or other requirements?

Can someone of severely diminished mental capacity vote? What about mental disorders like schizophrenia? Dimentia? Pathological anti-social behaviour?

And how would any of these things be determined or judged?

Thanks

As far as I know, you just have to be a legal citizen. My mother is mentally ill, she can vote.