Finding this out has been my principal hobby for seven years, and I still don't know the answer. I have learned a lot of Polish, though, as well as much information about memory training, bilingualism, cognition, and the imprecise definition of "fluent".
There's a lot of unsettled science, so I feel free to believe that some people are born simply more talented than others in learning languages, possibly because the two sides of their brain "speak more quickly" to each other. Of course, early training is very important, too. When I think about the multi-lingual members of my family, the big stars are the ones who were put in French immersion schools as children.
The biggest star, I suspect, is my sister Tertia, who is qualified to teach both French and Spanish, and is going to Italy in August for an Italian course. I am sure she will "smash it," as my Scottish spinning instructor would say. Tertia has everything going for her: early French immersion, a friendly disposition towards chatty strangers, years of experience teaching English abroad (mainly in Spanish-speaking countries), and God-given talent, whatever that is.
Occasionally I ask fluent non-native speakers of English when they could understand it all, and the most vivid answer came from a gregarious Polish nurse who said that one day a switch just flipped in his brain. I am still waiting for the switch to flip, but I have made gains.
The most astonishing gain is that I can now hear separate words in any foreign language, instead of just a mush of sounds. Somehow concentrating so much on Polish (and, to a much lesser extent, Italian) has vastly improved my ability to distinguish other foreign words, even when I don't understand what they mean.
The most useful one is that I can now speak my less-than-standard Polish without feeling horribly self-conscious, which is very important, because "fear is the mind-killer," as says the "Litany Against Fear" in Dune. This is thanks to my Polish tutor Anna, who says "Spokojnie" (Keep calm), whenever I am frantically searching my brain for some word I know I used to know.
By the way, it is so much better to find a one-on-one conversation tutor than to take a night class once you have the basic grammar down. For one thing, you get in more speaking practise. For another, nobody laughs at your mistakes. Occasionally fellow students would laugh at my mistakes, and this wasn't helpful. Feeling strong emotion about a mistake does help you remember the correction, says the science, but I don't think this works when you begin to panic every time you try to speak in class.
On Monday I was still in Poland and spoke almost as much Polish as I could, given that I was working in English on my computer much of the day. I didn't "have" to, of course, because Polish Pretend Son, Polish Pretend Daughter-in-Law and their real family all speak varying degrees of charmingly accented English. (PPDIL says her English is so good because she began learning it at 7.) However, it was a brilliant opportunity to be immersed, and I was grateful that at supper PPS, Mrs PPS and PPS's Real Brother all chatted together po polsku, even though I understood only about a third of what was said.
I did understand that PPS and his brother had plans to play football that evening, and that his wife was going to go along to watch and hoped I'd come with them. Naturally this was a cultural treasure not to be missed, so I toddled along.
"You'll hear a lot of bad words," warned PPS, and I certainly did, for about twenty Polish men gathered
on the village green by the town's five-a-side pitch, and they had to play in shifts. Various words one hears on Edinburgh buses came floating over to where Mrs PPS and I sat admiring PPS and his brother as they ran about after the ball. PPS's brother is a professional athlete, actually, so he can run a lot faster than PPS, who smokes and drinks, and I was edified to see PPS pass the ball to his brother as Mrs PPS squeaked encouragement from the seats.
"Is p****** a very bad word?" I asked PPS the next morning, trotting out one of the words I had learned during his game.
"Yes," he replied austerely.
"Whoops," I said. "Sorry about that."
This was rather amusing for it followed our discussion about dupa and kupa. My tutor Anna turned green when I once used the word "dupa" (roughly, arse), not being able to remember the proper word for "hips" (biodra, by the way), and when she discovered "kupa" (poo) in the children's story I was reading, she told me to stop reading it.
PPS said this was puritanical, and I have to admit that it is limiting not to learn relatively off-coloured language because it makes up a certain amount of colloquial speech. For example, on Tuesday morning before driving me to the airport, PPS said that he had kupa pracy to do that day, which roughly translates to "a lot of work" or "a ****load of work," as Catholic lady journalists do not say.
At the airport, I spoke Polish to all officials although I fell down a bit when I couldn't recall the words for "to the RyanAir desk" fast enough. (Note to self: "do kiosku RyanAir" would probably suffice.) As I stumbled, the young chap at the counter said in a cross tone that he spoke English, and I apologised in English, saying that I preferred to try to speak Polish in Poland.
"Of course that is very nice," he said in the same cheesed-off tone that doesn't necessarily match what the English-speaking Pole in Poland is feeling.
The fruit of this particular anecdote is that I didn't feel suicidal--as I certainly would have two years ago--but cheerfully went to the RyanAir kiosk to have my passport looked at and greeted the man there in Polish.
"In maybe about seven more years I will be fluent," I had said to PPS.
"Maybe five," he said kindly.