When I met my adviser for the first time, he gave me some math papers to read. This was in 1971;
personal computers did not exist yet, not speaking of Google translate:-) We spoke in Russian with my adviser.
I proudly said that I can read English and French, but not German. His reply was: "A mathematician has to read in all languages"!
So I spent a summer learning some
basic German. However my German is still weak, and I feel this has a negative influence on my research. Two years later my advisor gave me a paper in Catalan...
Later I moved to the US, and I know several English speaking mathematicians
who learned Russian to read mathematics. But the ability to read in 3 principal languages
(English, German and French) was considered a norm in my generation for all mathematicians.
French is especially useful, because after you learn French, you can also read math papers
in Spanish, Italian, Portuguese and Catalan, with a dictionary:-)
It is true, after 1980-s they are not writing math in German anymore, with rare exceptions. But so much interesting
math is already written in German! Very few books and almost no papers are translated into
English from German and French. And those translations which exist are usually very poor.
Recently I was discussing with my British colleague a translation of one classic Russian
book. He said that he does not really care, because he prefers to read math in the original language. He does not speak Russian and never took a course in it. But still he
prefers to read Russian math books in Russian.