The obvious question would be whether this works differently for languages learned as a child vs as an adult.

For me the "cost" of learning English as a second language in school was negligible (even if you factor in all the hours actually spent in class) because a lot of the learning was actually passive or even recreational (e.g. watching movies, playing games).

OTOH, learning a second (or third) language later on would have required much more investment. I actually tried several languages in university and eventually decided that I just didn't have the time or interest to learn any of them properly (i.e. the cost would have been too high and the immediate return extremely low with long-term return also being questionable).