[Orcrist]

If you really see yourself ending up in politics you are gonna have a hard time realising that in japan. To be part of Japanese politics you need citizenship. To get citizenship you need to live in Japan for atleast 5 years (without interruptions, correct me if Im wrong on this one tho). Even after aquiring citizenship you need to enter a political party in Japan, ofcourse this won't be easy. Now if all goes well, and you actually entered Japanese politics it will take a lifetime before you get a fancy position where you can actually make a difference on an international level.
So I'd say you will have to make a choice, either living in Japan or becoming an international politician. And as some people already posted, don't just go to Japan thinking you can easily last a whole year or longer there out of the blue. Do the exchange student thingy (im sure you can participate in many of the programs if you are really dedicated) for a semester of some sorts and see if you still like Japan as much.