I guess I’ll give a quick update on how things have been since this article was written. I haven’t used or studied French much since then, and have gotten quite rusty, though I’ve had a few conversations fully in French in the last few months and can still maintain a conversation. I have been thinking of picking up French again soon, as traveling to a French speaking country becomes more of an option.
I also did roughly the same thing with my Spanish in the following year, with similar results, though I’d say I probably reached a solid B1.5 or weak B2 after a year (due to COVID I was unable to take the proficiency test and lost some motivation to continue studying to a high level as a result.)
My recommendations are still largely the same, though I haven’t used these resources (e.g. Duolingo) since I stopped studying a while back. And as others have noted, for other languages, particularly those that are less closely related to English (e.g. Japanese), you’ll have to follow a different path to achieve the same proficiency. However, language learning resources are getting better constantly, so you can certainly still learn quite a lot without total, in-person immersion, as I did!