Huh? Renouncing first would make you stateless, which is against UN resolutions. I've never heard anything like this about Japan's naturalization process, just that you're required to renounce after acquiring it (and if you don't and they find out, they'll revoke your citizenship).
Many countries (including mine) allow you to renounce your citizenship but then void that renunciation if you don't acquire a new citizenship within 90 days, so as to prevent the risk of (long-term) statelessness.
