If by “strong” you mean visa free global access, Japan is tied for #2, behind Singapore.
How do you get proof of renunciation before becoming a japanese citizen? You would be stateless in between?
There is a range of options depending on a country, but it's some variation of two phase commit. Some places give you a citizenship certificate and a temporary id (paper one at that) that allows voting but does not allow travel and only gives out the actual passport once you give them the proof or renunciation (or proof of trying hard enough and not being able to get one for reasons outside of your control). If you don't come up with either or get the extension, the decision is annulled.
Some places give you and option of citizenship that is contingent only on renouncing the previous one. Here you can actually become stateless if you get the option, renounce the old one and never use it.
Some places just give a passport and a strongly worded letter to do or else.
It's of course ridden with loopholes added there on purpose, because life is complicated and operation manual for clerks that deal with it is ... something. I'm for example supposed to renounce my blue passport since I got a nice bordeaux colored one, but that obligation is suspended due to unfortunate circumstances outside of my control.
Sometimes you don't have to renounce if you are married to a citizen, sometimes your children get a free pass, but only if insist on filling in the form 42-2002/13477 which they don't like you to know about. Sometimes the cost of renunciation procedure or length of it are enough to pinky swear instead and send the passport to the embassy with registered mail and itemized contents bill.
Some people have two passports and only renounce (or promise to not use) one of them, thinking they outsmarted the system only to get it renounced 10 years later once they pissed off somebody more then they expected (the drama you didn't even knew existed: https://www.jpost.com/israel-news/article-910451 )
> How do you get proof of renunciation before becoming a japanese citizen? You would be stateless in between?
Usually they say:
“Here’s your new passport. You have X months to provide proof you renounced all your prior nationalities, otherwise we will revoke your citizenship. If you lie and we find out later, we will revoke you then.”
> Japan is one of those countries that doesn’t allow dual citizenship and they require proof of renunciation.
Japan doesn't allow dual citizenship if you acquire the japanese one later in life.
Plenty of people in Japan, like my nephew, have dual citizenship by birth. I don't know if they can keep both when they're adult. But as kids 100% guaranteed Japan allows it.
And then a great many mixed couple do, on purpose, give literally two different identities to their kids. One by registering them, say, in Germany with the name written in roman characters (say "Akane") and the other in Japan with the name written using kanjis (or kanas for that is allowed to).
I'm not talking about dual citizenships / two passports: I'm talking about the same person having two entirely different identities because his parents registered him, on purpose, differently in two different countries.
So, yes, there are literally a few people counted twice on earth. And you probably don't want to do that with the US citizenship (for US taxes its citizens worldwide and it's a reporting clusterfuck). But if you've got, say, a japanese identity and whatever european one: you're totally fine.
You are supposed to renounce your other citizenship by the age of 20. Not everyone does that, though, because it isn't checked.
They allow it until the kids are 19. Then they are supposed to make a decision about which one they want to keep.
Yes, but my understanding is you can keep saying you're working on making a decision and de facto keep both.