If you learn any language as an adult, chances are you're never going to become as fluent as a native speaker, no matter how long you live in a second language environment. I say that as someone who has done it myself, and seen a lot of friends and family going through the same process.<p>Children are a different story. They can learn a language and start speaking just like native speakers, local accent and all, within months. It's absolutely amazing to see. My niece went to Australia when she was 5. She could speak Portuguese, our native language, as a native, obviously, and was very afraid to learn English. A few months later she was fluent in English and had the local accent from basically day 1 :D. I never absorbed the accent myself (moved to Australia as a 23 yo, over 20 years ago). She is 11 now and she now mostly speaks English, and it takes effort for her to answer to us in Portuguese when we try to get her to do so.
Her dad, my brother, studied for years (even before moving) to try to speak somewhat fluent and correct English... but even now, more than 5 years later, he struggles. His wife barely speaks anything.<p>When I was new in the country, I could see that the younger the person, the faster and the easier it was to learn. No child I've seen ever had any difficulty learning a new language from scratch... they really are amazing at it!<p>I have been learning Swedish as I've moved to Sweden years ago, and I am in the same position as my brother is in Australia: I can only speak basic Swedish and I make mistakes and have to think a lot to say things coherently, specially without having to translate from English first - I've spoken English so long that it's my main language now... but I still understand Portuguese with less effort than English (and after just a few weeks speaking mostly Portuguese, as I do on some of my vacations, I become much better at speaking it again and words stop coming in English first, but that depends on the topic as well - if it's a topic I only ever speak about in English, like work, I find it horrible to speak about that in Portuguese!).<p>To think some people are able to learn 50 language (as mentioned in the post) blows my mind. I don't think I will ever speak my third language properly, let alone learn another dozen languages in the future :D<p>But this is another observation I've made over the years: a minority of people are very talented at languages and can indeed learn much more easily than others... though as far as I know, only children can learn within months and with perfect pronunciation.