What the Closed-door Policy caused in japan What the Closed-door Policy caused in japan

If you're specifically referring to the Sakoku policy (https://simple.wikipedia.org/wiki/Sakoku), you should say "Sakoku policy".

When you are a student, I recommend you to make friends with foreign people at least at least one foreign persone. When you are a student, I recommend you make friends with at least one foreign person.

学生時代に=when you are/were a student.

Having no friends from abroad is extremely unusual in a global society. Having no friends from abroad is extremely unusual in a global society.

In Japan, however, it is usual and commona common occurrence. In Japan, however, it is a common occurrence.

"usual" isn't really used like this. Rather than "X is usual", I would say "X is typical." I removed it here because the repetition felt a bit odd.

That is because Japan had been soused to be a closed society that almost no country bad nfew countries ever contacted. That is because Japan used to be a closed society that few countries ever contacted.

I would combine this with the previous sentence: "In Japan, however, it is a common occurence, because Japan used to be a closed society that few countries ever contacted."