Not exactly what you asked for, but this poll showed that ~8% or so respondents said their main reason for learning Japanese was that they lived in Japan.
More people may be living in Japan, though (e.g. I’ve been in Japan for close to 15 years, and odds are that I am here to stay, but that’s not my main reason to learn Japanese, so I replied something else). It also doesn’t show how many people would actually like to live in Japan 
My impression is that people would probably like to visit Japan, but not necessarily want to live here. Maybe you could make another poll in this thread, if you want to collect data?