I'm a second year medical student and the answer in short is yes, you need to be certified.
However, once you're done with med school you'll be so fucking sick and tired of school you'll never want to go back. You'll be old and grey and your shoulders will be collapsing under the massive debt you've accumulated--fat fucking chance you'll be moving into a country with lower pay just so you can watch animu and collect figurines.
Supposing for a moment that you, >>1, are in high school, my best advice to you is to focus on getting through 1) college, 2) the MCATs, and 3) the admissions process, and then figure out what you're gonna do after that--because most people, but the time they get to #3, have entirely given up on the idea of being a doctor. But if you really do have the patience, fortitude, and mental illness to do all that and still decide to practice medicine in Japan, more power to you.
Yours in anonymity,
Student Doctor Anonymous