I have seen pictures of prisons in Sweden, and no matter how old this particular prison is, I would not mind living there. They are literally posher than any room in my house.
In english (American), prison and jail have slightly different meaning. Jail is for short term detainment usually locally, while prison is for longterm detainment usually at the state or federal level. Is there a distinction similar to this Swedish or is "Fängelset" interchangable for both.
We usually say Han sitter i fängelse (if he's an inmate), but Han är i fängelse works too. You can't skip the preposition though.
Similarly in Russian we say "сидеть в тюрьме" - to be in prison, as well as many derivatives, such as "отсидка" (confinement), "сидевший" (prisoner) and some others
I tried to input fängelset with "jailhouse" but it was marked incorrect, the correct term offered being gaol. Would prison do?
Yes, and "jailhouse" is perfectly fine as well, so I've added that solution now. :)
Fun fact: The former prison in Falun is now a hostel. I once stayed there and while the rooms and service were nice, it did feel awkward to have those heavy doors close behind you.