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.

It isn't the same thing, if you try to speak poor Finnish in Finland they just switch to English since most people speak English. In Japan most people don't even understand basic spoken English so getting them to speak Japanese with you is really easy.

Finnish kids have finnish-speaking parents, and spend their entire days immersed in the language, at home, daycare, or school.

Expats are most likely are in an English-speaking environment at work, where speaking Finnish at the level of a four-year-old is going to be a bit of a hindrance; no practice at home unless they have a Finnish partner, and no real options for social activities that will tolerate the learning curve, like children have.

And if it is anything like the Netherlands, any mistake you make will instantly and irreversibly switch the conversation back to English.

I moved to Finland, and then had a child. He's now nearing four years old and he's very bilingual - to the extent that he translates for me at times.

Children learn the local language, partly because of immersion, but also because they're corrected nearly constantly when they begin to speak. We forget that when they're capable of "good" communication.

I'm seeing interesting things here, I speak to the child exclusively in English and as far as he's concerned I speak/understand zero Finnish. I'm hoping I can start speaking more Finnish in the near future, he should be able to understand I'm "mostly English".

One obvious thing that really drives the language home is the notion of pronouns. Finnish has no gendered ones, so when he speaks English to me he'll be "Mummy is asleep, he will wake up soon?". I have to keep saying "Mummy is a girl, we say she". I've been doing that for 8+ months, and he still doesn't get it right. That's the level of repetition that's involved in learning a new language.. and even now it hasn't "stuck".

Finnish is notorious for its complex grammar. I'm half-Finnish and still cannot hold a real conversation even though I took classes for years when young. For Japanese one semester was enough to be able to hold a basic conversation when visiting Tokyo.

If anything Finnish is much easier than Japanese for Europeans.

Only if you're from Estonia or Hungary. Finnish is so different from all the other European languages that it's like starting from zero. The letter system aside I've found Japanese much easier to learn and I'm half-Finnish (Swedish being my native language).