The problem with software engineering in Japan is that the salaries are much lower than in the US, though they're not too awful compared to Europe.
Top talent from, say, Vietnam already speaks good English and many would jump to the opportunity to work in Japan under these conditions.
I haven't met too many Vietnamese though, but I have met a couple. Japan also attracts a lot of Filipinos (though not so much in the IT sector, in my experience).
Also, the entirety of Japan isn't relatively wealthy, just like the US. The rural areas are dramatically cheaper to live in (for housing mainly) than the big cities. However, there's less "spread" than in the US: all the software jobs are going to be in a handful of large cities, not in some remote prefecture or village (whereas in the US, there are jobs in some smaller and even very small cities, where that one employer is likely the only place you can get such a job nearby and you'll have to move if the job goes bad). But even if you just look at Tokyo, the pay range can be enormous, at least a full order of magnitude.
Mid-level probably 5-10m.
That’s just based off what I was seeing when I was interviewing and considering a move there. Ultimately decided against it.