The law is somewhat vague. You have a "duty to make efforts" (努力義務) in rescinding your foreign citizenship after electing Japanese citizenship. The Ministry of Justice has never clarified its interpretation of 努力 (i.e. what counts as "making efforts"?) I am friends with a few Japan-US dual citizens who have done nothing after electing Japanese citizenship; they are essentially gambling on a gray area of law.
Note if you acquire Japanese citizenship afterwards (or never had a second citizenship in the first place), then 国籍法11条1項 automatically voids your Japanese citizenship at the time you obtain a foreign citizenship.
The Japanese law attempts to require citizens to hold no more than one citizenship and return any extra passports to your countries of origin, as would seem to be the most reasonable to Japanese laypeople. OTOH, recognizing a citizen is within sovereign rights of a nation.
So, for the Japanese government to a person to hold no more than one citizenship is beyond the rights held by the Japan as a nation. It cannot demand e.g. the King of England to revoke citizenship of a British subject, just because that guy is currently in Japan and on its own family register. If the UK's HM Government recognizes that guy leaving and converting, and amends his status accordingly, voluntarily, then that is just friendly international coordination and cooperation, and the sovereign rights of the UK is not violated.
And therefore, Japanese laws merely state its official stance of not allowing dual citizenship, and require applicant to make efforts, but it will not require the results of the effort to be exactly as it desires.
In an ideal world, there can be something like a "UN global citizen register" for every person on Earth, and the current nationality and/or recognized available nationalities can be fields in the global koseki printouts that can be printed out at a 7-11 everywhere. That would be very friendly to Japanese mental model of citizenship, but we're still maybe a century from there.
> they are essentially gambling on a gray area of law.
It's not so much a gray area if they didn't do anything at all. The translation in English isn't on a vague side of legalize. One needs to go and try to renounce citizenship and keep the receipts, so if the motion is denied or you get sucked into a bureaucratic hell, you have a proof that effort has been taken, but the bureacracy prevailed.
It's interesting, that the word "effort" doesn't have a qualifier here, like "reasonable". Hazard guess -- the general language of the law somehow implies it, that if renouncing citizenship costs 1mm USD, you don't have to try unless it's pocket money for you or at all. If renouncing takes flying half the world to a country where things are happening , something should say ah nah.
