My experience is the opposite. I don't have to unlearn native patterns. I learn a different pattern, and switch to it. Because I understand grammar and the parts of speech as an adult, at a level beyond simply having copied it from people around me, I can understand foreign, completely different grammar, at an intellectual level.
When I speak (and learn) Japanese, my conscious understanding of Japanese grammar simply makes it easier. The same when I speak French. I don't unlearn anything; I learn different patterns, and I simply switch to them.
In fact, it's what you suggested about programming languages, but for spoken languages. In my experience, adults can learn foreign languages far faster than children can. Maybe it appears that children are better at it because they are completely immersed in the language they need to learn and have no other means of communication. If I spent 16 hours a day immersed in a foreign language, with people trying to communicate with me and no other way of communicating back, I reckon I'd pick it up pretty swiftly.
Interesting. I admittedly don't have much experience learning foreign languages; I did K-12 in the US and was only required to take 2 years of Spanish. I'd assumed from discussions with other students taking, for example German, that it'd be more difficult. Versus programming languages, I've learned, C, C++, C#, Java, Python and Perl (and I supposed a modicum of JS) as well as x86, x64, 68k, MIPS, z80 and Atmel AVR assembly. I've always had an easier time learning a new programming language/assembler than a foreign natural language. Maybe that's just me.
Spoken languages are a lot harder to learn than programming languages.
But think about how much faster you learned JS than the time it took to learn C (or whatever your first programming language was.)
Don't forget to account for the vast amount of learned information you get to leverage from your first language, even if you have to unlearn a few things or learn a few new things.
Even though JS is very different from C, learning JS was much, much faster for me than learning C was, because I already knew a whole lot about the way all languages work just from learning C.
I'm learning Icelandic right now, and it only took a few weeks of 30 minutes a day to cover some basic conversational skills, gather maybe 100 or so words of vocabulary, and get an introduction to the basic speech patterns & conjugation. It'll take a lot longer to be fluent, but children take years to get that far, because they're starting from scratch and don't have connections to build on. I see a ton of connections to German and English that kids wouldn't.
It's also hard to compare programming to high school Spanish, since it's easy for us to spend 10 hours a day programming, but in high school we topped out at like 3 hours a week of Spanish exposure, and most of that wasn't practicing very hard.
I moved to Mexico as a child (during my first grade year), and learned enough Spanish to go to school, play with the kids, etc. I feel like I'm picking up languages faster now than I did then.
YouTube anecdotes, but here is Gabriel Wyner: https://www.youtube.com/watch?v=iBMfg4WkKL8 describing his experience of learning nothing much linguistically in High School or College, but then joining immersion based courses at an adult and making significant progress in a few weeks.
and here: is Chris Lonsdale saying adults can learn useful amounts of spoken Chinese in 6 months, with immersion and being able to watch mouths closely and see how speakers move their moouths to pronounce things (IIRC): https://www.youtube.com/watch?v=d0yGdNEWdn0
When I speak (and learn) Japanese, my conscious understanding of Japanese grammar simply makes it easier. The same when I speak French. I don't unlearn anything; I learn different patterns, and I simply switch to them.
In fact, it's what you suggested about programming languages, but for spoken languages. In my experience, adults can learn foreign languages far faster than children can. Maybe it appears that children are better at it because they are completely immersed in the language they need to learn and have no other means of communication. If I spent 16 hours a day immersed in a foreign language, with people trying to communicate with me and no other way of communicating back, I reckon I'd pick it up pretty swiftly.