I also learned Japanese to a professional level very rapidly by immersion (and focused study in parallel), but I was single and not working at the time, which was critical. If you have a full-time job (that you need to perform in your native language), you lose 50% of your immersion potential; if you have a (non-local) family, you likely lose the other 50%; now you are just spending scraps of time studying.

So I think in this context of a professional—who likely has a family—coming over to work, deanclatworthy is right.