For Germany:
An inmate has a right to have objects for training or leisure activity.
This right is limited in that it may not endanger safety and order of the penal institution, or require undue activity on behalf of the institution to ensure this safety.
A computer is considered endangering that safety, due to its capabilities to record, store, and / or transmit information (let's say, about guards, facility security etc.).
This can be judged for the individual case, if there is some extraordinary need on behalf of the inmate, but
generally speaking, you won't get a computer in your cell. No mobile, either.
You
can use a shared computer, under supervision, the same way you can use e.g. a phone.
(ref.
Federal Constitutional Court, 2 BvR 1848/02.)