One of my favorite comfort video content on youtube are the ones where polyglots travel around the world's most popular tourist spots and talk to the tourists in their native languages. The faces are worth watching. What is more interesting is how smoothly do the polyglots switch from one language to another without any hesitation. It is like second nature to the polyglots.
I speak almost 8 languages. 5 of them at a native level. I can write and read in all of the 8 languages. I usually get asked how did I do it. How did I learn them? And to be completely honest, I have no clue. I was privileged to be surrounded by people of different languages. And conversing with them on a daily basis gave me the unique opportunity to pick up the languages. There are only about 3 languages in my arsenal that I voluntarily put them effort into learning.
As I grew older I started loosing touch with most of them. My circle of friends became smaller and slowly but steadily I felt I was losing my touch with the languages. There was a time when I couldn't speak most of the languages. I had simply forgotten them. Just like the clouds on a sunny day, the languages vanished from my brain.
See, languages arent like riding a bike. There is no muscle memory involved except the movements of the tongue and voice box. The language itself is something that cannot be retained as a muscle memory. It needs practice. A lot of it. When I realized I lost something that many people didn't have, something specially unique, I once again found ways to practice and regain my hold of the forgotten languages. Soon enough, and now, I am back to speaking all of them.
I have seen bilingual people who have been speaking 2 specific languages since birth forget one of them. It is nothing new. There are tons of skills that can be easily forgotten. Unline playing the piano or riding a bike, it doesn't reappear when it has to. I have a friend who forgot english. English! A language we use almost everyday. He forgot most of it in the quarantine period. He was locked in his room, and watching native shows all day. Lockdown ended, and English went out the window.