When did English come to lead in published research? As any modern student knows, nearly all published research these days is in English. That's just the way of the world. But it didn't used to be that way. Many of the most pivotal pieces of scientific literature in the world, like the writings of Riemann, Cantor, Einstein, Galois, Schrödinger, (Gregor) Mandel, Linnaeus and many, many others were written in German, French or Latin. These days, publishing in a language other than English hurts your credibility, and is largely discouraged.

When did English become necessary for publishing scientific literature?
English became the dominant international language of science across the first half of the 20th Century.  

To summarize an excellent BBC article on the subject:


At the beginning of the 20th Century, there were three main language of science, French, English, and German, essentially corresponding to the biggest economic and political powers of the time (England, France, Germany, and the USA).
After World War I, Germany and the German language were essentially ostracized by the victorious nations, including an (unconstitutional) ban on the language in the United States. Europe was also generally weakened as a power.
The continued devastation of World War II and the rise of the US as a technological superpower cemented the collapse of German. French took longer to decline, but network effects and relative power had their impact eventually.


Thus, by the middle of the 20th century, English had become, in general, the dominant language of science. It was by no means absolute (and still is not), but its dominance has further strengthened over time. How long this will last and what will replace it are for anyone to guess.