Speak for yourself. You're probably a native English speaker and have it easy.

My school taught Indonesian. It was a very popular complaint among students that we should be learning a more ubiquitous language like French, German, Japanese, Mandarin, or Spanish.

The only thing I know in Indonesian is ular besar (big snake)

Language classes in school are horrible. You're going for an hour a day for 180 days, with significant gaps every few months, and moving at the rate of the slowest learners in the class.

500 hours of constant, immersive study would likely get you most of the way there, which is not the same as being immersed for 500 hours without study :)

I thought I was doing well with Duolingo once, then realized, 40 hours in, that I had almost no concept of formal/informal, and barely had any verb conjugation or grammar.

If your main language is English you probably can't. There's just little need since everything is so English-centric that almost everyone else has to learn it as a necessity.

Larger countries like France and Germany can often get on without it as there's enough population to be worth dubbing and translating things to it, but go somewhere smaller like the Nordic countries, and you're basically stuffed without it.

I've had French and German for 13 years. I don't speak a word of French and can say very basic things in German, but not complete sentences.

I took one semester of Russian in uni and I still can read the Cyrillic alphabet

Have you tried using it?

I studied a book going over "chagatai tili" in highschool.

I remember absolutely 0 words or grammar.

I got out of the language requirement in college by taking computer science courses, which counted as "language" only because programming languages are called what they are. It is just the dumbest fucking shit. If they were called "paradigms" or "code instruction sets" or something like that (which would be just as or more accurate than "languages") it never would have occurred to anyone to let us computer nerds -- who are already not exactly well-rounded in general -- to get out of learning a real fucking language.

I studied French for six semesters. I kept failing.

They wouldn't let me take foreign language classes because my English grades weren't good enough. It wasn't that I was even bad. I just didn't do homework.

"Mexican Americans....take Spanish in school...and get a B"

I took German in school, then moved to Germany and gained (rudimentary) fluency, then moved back to the US and lost it after a couple decades of disuse.