"国について真に知っている人はいないと言われています、一度その国の刑務所に入ってみない限り。"

Simplified Meaning:

To really understand a country, you have to look at how it treats people who have broken the law. This is because the way prisoners are treated shows what a nation values and how it handles justice and fairness. For example, if a country's jails are in terrible conditions and prisoners are treated very poorly, it might mean that the country doesn't care much about human rights. On the other hand, if prisoners are given a chance to learn and improve themselves while in jail, it shows that the country believes in second chances and rehabilitation. So, visiting a country's prisons can tell you a lot about the country's overall values and priorities. People can use this idea to remember that the true measure of any society is how it treats its most powerless members.