Prisoners violate the rules of society, so they do not get to make decisions about society, such as political decisions involved in voting. Internationally, there is no support for prisoner voting rights. In fact, most countries grant convicted prisoners even fewer civil liberties and rights than the United States does.
I am of the old school if you will. I believe that if you have committed a crime that has landed you in prison for any extended period of time, you are a criminal, and you have given up your right to vote either on a local, or international level.