Here's something interesting I'd like to see: Take 1,000 random expats living in France for at least 2 years and assess their French speaking ability and then do the same in Poland and see what you get. I would bet all the money in the world that the expats living in France would be able to speak, read and write WAY better than the expats in Poland. Another assumption I'm willing to make is that the assessment results in France would show close to zero people with no French speaking skills at all yet in Poland, there would be a significant number who can barely string a sentence together.
No, to practically everything here. Do you need to use Polish all day every day in your job? If no, then you're spending most of the day speaking English, which is not immersion. If yes, then you must know Polish a lot better than you think, at least what is required for your job... which would I imagine be considerably more than 'decent'.
Japanese is NO more difficult than any other language, barring having to learn a new writing system it's no different to learning any other language. I've known a person who has tried to learn Spanish but could never get to grips with it, who later has gone on to learn Japanese to complete fluency (Kanji too) in just over a year. The language itself isn't really so complicated, and all 2,500+ Kanji can be learned in 3-6 months. You can claim it's all down to her being a 'talented linguist' all you want but the fact remains; she struggled with supposedly the worlds 'easiest' language. Another friend of mine managed to learn huge amounts of Czech in little over 3 months of immersing himself in Prague. So I don't understand why exactly you seem to think learning any other language would be easier for you... have you tried? How many languages ARE you fluent in?
As for your experiment, it makes no sense. Firstly, how many expats in either country have jumped in with both feet to learning the language? How often are these people speaking English and not French or Polish in their free time? And finally, even if it is shown that more of the ones in France speak better French than the Polish learners, how do you prove that it wasn't just the better and more widely available resources for French learners?
Out of interest, what things can a child do that an adult cannot learn in a short time? Assuming you know by now that the idea of children having a natural ability to learn languages faster than an adult has been utterly disproven...
It stands to reason that if people no more talented at learning languages than anyone else here can learn Japanese, Russian, Hungarian, my own native Irish, Chinese, Hindi, Farsi, Kurdish or even one of supposedly the most 'difficult' languages of all; English, in under 2 years, then it stands to reason that Polish is nowhere near as difficult as some on this forum seem to like to make it out to be.