>In my job, in foreign-language education, I think a lot about what motivates some adults to keep studying long and hard enough to become proficient in a foreign language, as relatively few actually succeed.

I'm not sure I would compare something like bodybuilding with foreign languages. At some point you become proficient enough in a language that you can simply consume movies, TV shows, books, news etc that the language learning becomes mostly effortless. Becoming really proficient at a language requires someone to immerse themselves in it (to use it for a purpose without easy fallbacks). The internet has been amazing for learning English.

I don't know anyone that became proficient at a language by studying it. It's always been through use. The studying just creates a base that allows a person to start using the language.

Exercise requires persistence. You can't really have gaps in it, because you lose your progress. Language learning is much more flexible in this. You might forget what you learned, but the next time you try to learn the same thing it's much easier.