I'm very curious about what makes you think that. I belive it's true only if you already speak one or two other similarly rooted languages, or if you learn the language before you're 10.
Did you learn your first foreign language as an adult and found it "not particularly hard"?
this also depends a lot on the language, many of the Romance languages are relatively easy to learn, many of the Nordic ones seem quite difficult.
Difficulty of course may depend on what you are moving from to what you will be using.
Leaving that aside, there is a concept of linguistic distance from people´s first language, which makes it easier or harder to learn another language. French and English are very close to each other, as are Arabic and Hebrew, the Scandinavian languages are also all very close to each other (a little further from English), except for Finnish of course, which is kind of out there on its own. There is a nice recent paper looking at this in the academic context:
https://www.sciencedirect.com/science/article/pii/S004873332...
Essentially how far your native tongue is from the language you are attempting to learn will drastically affect how long it takes to learn it for most people.
Some languages take english L1 speakers 3x as long to learn as others.
I agree with GP's 2 years for Cat I-II languages (assuming you're mostly living there and learning the language in your spare time, not intensively).
I've forgotten almost every word. It would be quite difficult for me to relearn, at 61, and I'd likely not have anywhere near the efficacy, that I had, then.
People say how easy it is to learn new programming languages. I've probably written in a half-dozen different ones, over the years.
IME, learning the basics takes a week or two, but it takes years to really grok the language, at the fundamental level.
Have you tried? It would probably be a lot easier than you imagine.
It would not be useful, unless I lived in an area where I would use it. Maybe if I moved to Canada.
Same goes for computer languages. At one time, I was quite fluent in C++, but I can hardly recognize it, anymore.
If you remembered pre-C++0x, or god forbid, pre-Standard C++, it would probably hurt as much as it helped, as your old idioms would either be outdated by new std features, or outright dangerous by modern coding standards.
At least with old spoken languages, you'll just either sound like a small child or a character from an old movie, using slang that no one uses anymore.
Having said that, this is a thing to be able to learn a language enough to interact with people and live in a country where another language is spoken. It is a different thing to use that language to understand completely scientific/medical/law/tax forms and texts.
Jargon has to be learned independently. This is true of every kind of jargon, not just academic and legal stuff. If you want to talk to junkies and sound like one of them, you'll have to learn how first.
The technical term for this kind of concern is usually "register", as in "writing in an academic register".
A Chinese college student once asked me to review a paper of theirs for English quality, because their professor had criticized the English in a prior paper and they trusted me to be a native speaker (which I am). But being a native speaker didn't really help; once I saw the paper, I had to say "I'm sorry, but I don't have academic business training; I can't guarantee that anything I said would be phrased correctly."