How do you know it was the Japanese version?
1. Because it is hard to get this Visual compiler in another language version, including English.
2. There were some bugs, related to ¥ character in paths, which usually happens, if you use Japanese version of Windows:
https://en.wikipedia.org/wiki/Yen_and_yuan_sign#Microsoft_Windows3. You have to use Japanese system to handle Visual in Japanese version. If you have English system, it will display strange squares, or not render the font at all. Also, the language is integral part of the program, and there is no option to change it easily in some menu, so you need both software in the same language (the system and the compiler).
Satoshi's whitepaper shares notable similarities with Nick Szabo's writings though.
Their style of writing text is completely different. For example, Satoshi is known for using double spaces as a separator, instead of a single space, between sentences, after each dot. Another reason is the whole construction of each text, where Satoshi starts creating it from the bottom to the top, while Nick simply uses top to the bottom approach. Also, this is another reason to read the whitepaper in the right order: Satoshi first tested hash functions, and calculated probability for chain reorganizations, and so on. The same pattern can be observed for his posts on forum.
Perhaps Szabo penned the whitepaper, while the technical design originated from a different mind.
I am quite sure that everything was done by a single person. If you ever tried to coordinate any group, then you probably know, what am I talking about. Even when I am trying to create something with Garlo Nicon, it is sometimes hard to plan our actions in advance, and when the group is even larger, then it becomes harder and harder.
If Satoshi would be a group of just two people, it would create a lot of different patterns, than what you can observe. It is much more common to split a single identity between many accounts, than to merge more than one person, behind a single account. Been there, done that, the initial idea of the Garlo Nicon group was to publish all posts under a single name, and it quickly failed, so we registered separately.
For example: imagine that there are two people, and they disagree about anything. The first person wants 10 minutes per block, and the second one wants 15 minutes per block. One wants "explosion of special cases", and another wants "the Script". One thinks about "simple for loop with additions", and another wants to "optimize every bit of that, and do right rotation". Even between Bitcoin developers, there is a huge space to disagree about some details. How do you imagine to handle all of that? Who would be in charge, and decide, what is published or not?
And there is another problem: if there are more people, then what happened, after Satoshi disappeared? All members of the group died simultaneously? Nobody wanted to take the lead? And what about private keys? They shared all of them? What about GPG keys? What about all kinds of accounts? Having a group of just two people leads to a lot of problems, to do all of that correctly, not to mention about any bigger group.
Also, when it comes to the amount of work, produced in a given time, if there would be just two people, then they would produce more content, and you could catch it immediately. Which means, that if you have any larger group, and you want to pretend, that you are a single person, then you have to introduce delays in publication. And even then, to reach a huge level of agreement, you need some kind of coordination, like putting everything through a single person, before publishing it, to not introduce some small and subtle changes, which can be used to easily separate each author.
