Well, maintaining your current level of strength is far easier to do than maintaining continuous progress (especially at more advanced stages, which you have to do a lot of fuckery around programming/scheduling in order to realize any progress at all). You just can't do nothing, but the same goes for languages. If you don't use a language at all, your proficiency atrophies over time as well.
It's just not something people really think about. If you were to reach a level of strength you were happy with, you could just figure out a maintenance program and focus your physical energy on other pursuits, extreme sports, slacklining, bouldering, hiking, whatever.
I don't think it's quite comparable. I was fluent in German more than a decade ago. I've not really used it since. It's difficult for me to speak or write in it, because I simply can't think of the words - I'll remember them in English instead. However, if I listen to German or read German text then I have no trouble understanding it.
This same thing did not happen with exercise for me. The strength I once had is gone.
TLDR, your muscles, unlike most tissues, have more than one nucleus per cell. Your "meat", which are protein fibers, can be atrophied through catabolism during diet or dis-use, and this is a form of protein recycling. But the multiplication of nuclei which occurred during your original training does not revert.
So a formerly strong person who let it go is sitting around with a high nuclei/cell ratio, and low muscle tissue. If he resumes his training and calorie surplus, his nuclei will act like parallel processors outputting the commands to manufacture new muscle fiber much faster.
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.