I want to learn so that I can read/understand publications in mathematics in a foreign language, mostly Swedish, French, German. (*) For this exercise, the typical apps do not help much.
(*) I would have liked to add Latin and Greek too but that's mostly a pipe dream.
Reading old mathematicians and scholars I have realized something that runs quite counter to the common perception we have, especially in my country.
That common perception is school kids, especially in mid and high school are overwhelmingly burdened by sheer volume of subject matter to learn at a very young age. But then I look at educated teenagers from 17th - 18th centuries, who went on to become mathematicians or scholars, they were so immensely well read at a very young age. I understand this is a biased sample, but many of these people, Newton, for example, were ordinary folks (socio-economically speaking)
Hamilton (I concede that one cannot compare Hamilton with a typical modern teenager) was already quite fluent in thirteen languages in his pre-teens. Apart from the usual suspects, he knew Arabic, Hebrew, Farsi, Sanskrit, Hindi, Marathi.
This might sound atypical but this was not unheard of. One of the poets in my language was fluent in Hebrew, Greek, Italian, French, Latin, Sanskrit, Telugu, Tamil, Bengali, English.