Interesting, you are learning languages for math publications but you didn't include Russian ? unless of course you are native (or you are from ex soviet)

That's a great point because a lot of the maths literature I am interested in is actually in Russian (optimization, probability).

Thankfully there is the "Translations of Mathematical Monographs" book series

https://bookstore.ams.org/mmono

I had just resigned myself to the fact that I will probably never be able learn Russian. At an optimistic best, perhaps French and Swedish only, if at all.