Judging from the language which the client was, I can fairly say that Satoshi Nakamoto is not a native of Japanese but just used the pseudonym so as to conceal his true identities. Also, he asked people if they can translate the client into different languages, and if Satoshi was a native speaker of Niponggo, then he would probably do the translation himself. Also if you can read his posts in this forum and other websites, his proficiency in English is outstanding and is very profound (and if you note the time of his activities in his forum, he may be living somewhere in UTC +0, British maybe? Or just having a strict sleeping routine).