You'll need an (unfrozen) ICAO ATPL to apply for a job in Japan even for an FO position. You'll then be spending the better part of a year for ATPL conversion upon joining an airline. Your airline will provide you with the conversion course. Yes, you will have to sit the local Air Law exam and the JCAB oral exam which is notorious for containing some outrageous questions. If you are interested in finding out more about the conversion process and flying in general in Japan I'd recommend Secrets from the Cockpit by Robert Schapiro.