I took German in high school and forgot it all immediately. A decade later I found myself in India studying Malayalam, the language of Kerala which is the southern-most state in the country. Very hard language to learn but as I was studying its formal grammar I was like, wait a minute this is very familiar. Turns out a German monk in the 19th Century visited Kerala and gave Malayalam its first formal grammar, which was basically just German’s grammar. So it wasn’t totally useless.