Since you have not well known the field that interest you most, I would recommend you just hold on a moment to have a travel for 1-2 years overseas to explore your interest at first before you really determine the subject to study in the university overseas. 

Honestly, I didn't have such experience for studying English aboard. But I can advice you to speak more to people there. For language exercises , you can do it even your country (Japon). but speaking and meeting  native speakers is an opportunity for you that you have to chase it. So you have to practice and practice with all people not just host family, but you can also visit different places where you can meet new people  and exchange topics together, have fun english and don't forget to take their contact to continue your practice with them after your coming back Japon.  I think that's the best way. Don't focus just on language structures and exercises. Enjoy your fly!

I would advise concentrating a bit less on the actual studying and trying to hang out more with foreign students. Do something fun together and trust me your English will get much better than from just attending classes!

Okay I'll try to make foreign friends and communicate with them!! Thanks a lot!