German was a language of science, but I don't think it was ever the language of science. It varied by different fields — in the 19th century, the language of chemistry was German, but physics was in German, French, and English. At the time you are describing, the main languages were German, French, English. If you published in a different one, you couldn't expect to be read; if you couldn't read those three languages, you couldn't keep up with the literature.

My friend Michael Gordin has recently written a book on the changing language_s_ of science. He also wrote an article recently for popular audiences on the topic. His work is super interesting and goes into interesting places — people who wanted to use Esperanto as the language of science, fears of Russian dominance leading towards the first development of machine-translation software, and the many Cold War forces that contributed to English becoming the lingua franca of science today. He also did crazy things like learn practically every language in the book, including Esperanto. Give it a whirl.

As for your specific claims — yes, there were concerted activities after WWI to reduce the impact of German as a language on science. It was one of many politicized instances regarding the language of science. On German, he specifically writes:

After the war, the International Research Council, formed under the aegis of the victorious Entente – now including the US but excluding Russia, which had descended into the maelstrom of the Bolshevik Revolution – initiated a boycott of scientists from the Central Powers. New international institutions for science were erected in the early 1920s locking out the defeated Germanophone scientists. This exclusion lit a long-delay fuse that, in the coming decades, would contribute to the death of German as a leading scientific language. Three languages had, for part of Europe, diminished to two. Germans responded to their predicament by reinvigorating their commitment to their native language. The multilingual system was beginning to crack, but it was the Americans who would shatter it.

In the Germanophobic frenzy that followed the entry of the US into the war in April 1917, German became criminalised. Iowa, Ohio, Nebraska and others rolled back what was by far the most commonly spoken language besides English in the US (a consequence of massive immigration from central Europe). The proscription of German only grew after Armistice Day. By 1923, more than half of the states in the Union had restricted the use of German in public spaces, over the telegraph and telephone lines, and in children’s education.

That year, the Supreme Court overturned these laws in the landmark case of Meyer v Nebraska, but the damage was done. Foreign-language education was devastated, even for French and Spanish, and a whole generation of Americans, including future scientists, grew up without much exposure to foreign languages. In the mid-1920s, when German and Austrian physicists published about the new quantum mechanics, American physicists were only able to read the German papers because Yankees still traversed the Atlantic for graduate study in Weimar Germany, and had necessarily learned the language.

As a 20-year-rule-breaking aside, my PhD program in history of science still had a two-language requirement, with German and French being the default choices, because of the idea that you wouldn't be able to keep up with the literature without them. There is considerable uncertainty how relevant it is anymore. (I substituted Russian for French because for my area of study it was more germane.) I have heard of programs that have begun to allow students to substitute proficiency in a programming language (e.g. Python) for a foreign language, which I find both interesting and terrible. (As a programmer I thoroughly encourage the learning of programming languages, but I don't think they should count for language requirements — either abolish the language requirement or don't, but let's not pretend Python is a real human language. It is a limited subset of English that is used to express programming logic, and it is the logic, not the language, that actually matters for programming aptitude.)

This article confirms the statement.

It is noteworthy that German was never the scientific language the way that English is today. Shortly before WWI English, French and German were equal in terms of number of published articles. The boycott of German scientists and the widespread movement to suppress German in America were a considerable factor in the decline of German as a scientific language and the brain drain before WWII contributed again.

After WWII it was the enormous amount of money that was funneled into higher level education and R&D that made English and to be more specific American English the lingua franca of science that it is today.

While it may not have been a lingua franca of science in general, in chemistry it very much was the language. Up into the 1950's, if you were studying chemistry in an American university you most likely had to learn German as they had essentially built the foundations of most of the modern chemical industries and had been at the cutting edge in chemical research for most of the 1900's, in large part inspired by the German military's need for natural resources that could be produced in house.