The idea of having to REGISTER to vote is very foreign to people in Sweden.

In Sweden and many other countries you are automatically registered to vote when you turn 18 (or of legal voting age) and never have to think twice about it.

And yes, people in prison can vote as well.

I don't know how widespread this knowledge is in countries where you have to register.

Automatic eligibility to vote of course only works in countries that enforce mandatory residency registration.