Now that [the texts of all of the Plenary talks][1] at the international congresses of mathematicians is available online, that could serve as a data source for the transition. This year, every plenary talk was in English, and none of the speakers (that I attended) were at all difficult to understand on account of the language.

As a spot check, in 1978 several of the talk titles are in cyrillic (presumably Russian), and at least one in German.


  [1]: http://www.mathunion.org/ICM/