[jeena]

Or because the Germans lost WW2. It would be interesting to see another time-line in which they didn't just to test my hypothesis.

"... German advanced to become one of the most important languages of science and scholarship, and at the beginning of the twentieth century ranked above English and French, especially in the sciences. In order to keep abreast of the latest developments, scientists and scholars all over the world learned German, a circumstance that contributed to German’s becoming one of the most widespread foreign languages. With the end of the First World War, the “primal catastrophe” of the twentieth century, a reverse tendency arose. Germany and Austria had been economically ruined by the war and were in no position to invest in research and knowledge to the same degree as before. ..." (from http://www.goethe.de/ges/spa/pan/spw/en3889454.htm )

[riffraff]

while I don't have access to another timeline, russia, china and (to some extent) france did win the war, but none of their language became a new lingua franca.

I'd suppose the closest thing to a nazi-controlled-germanized-europe would be the USSR (people escaping, restrictions on freedoms hindering creativity, no colonies speaking the same language, trade issues with the rest of the world) which in fifty years failed to establish a linguistic dominance even in aligned countries.