The countries where everybody is multi-lingual this almost assuredly happens in middle to early childhood.
However, I believe many people overestimate the advantage children have in learning a language. For example, a four-year-old can have a basic conversation and communicate their needs, but they are not yet able to have a complex conversation. Their vocabulary and pronunciation may not be fully developed yet.
It's important to note that this child has spent a significant portion of their life learning the language and is exposed to it daily, and likely has parents or others to practice their language skills with. Despite this, their language level is not yet perfect.
The point I am trying to make is that it takes a significant amount of time and effort to learn a language, and there is no shortcut or "hack." Most adults would likely do comparably well if they dedicated the same time and effort to learning a language as children do.
The fact is any adult can learn any language, they just either don’t have to or don’t want to. I’m not saying it’s easy or it’s the best decision for every situation, but it’s definitely possible. This is true even more so if it’s a popular language with a lot of resources to learn it, and you live in the country where practice is virtually free.
I don't want to be too presumptuous here but my understanding is there are a lot of desperate broke African immigrants finding their way into Italy.
In such a situation you would be forced to rapidly acquire the language to survive, move on somewhere else, or possibly resort to crime because you are unable to communicate to secure a job (which <should> rapidly leads to expulsion from the country). Bad learners get tossed into the last 2 bins, and thus unseen.
Comparatively if white American hacker guy shows up flush with tech cash there's really not much chance I'm going to find myself on police radar or starve not learning the language, even if I'm there for an extended period and even if there illegally.
Probably, but I don't think it challenges the legitimacy of the original point at all.
> Comparatively if white American hacker guy shows up flush with tech cash [...] not much chance I'm going to find myself on police radar or starve not learning the language
True, but you might not have a very good time.