Here's an intermediate answer:
ICAO Annex 10, Vol. II, Chapter 5 lines out the phraseology of R/T, but it doesn't specify English as the language to be used.
A.I.P. Canada 5.2 says that both English and French can be used. (CAR 602.133 134 and 135) It also says that once a language has been determined, the pilot should refrain from changing language [...]
Let's take a look at Mexico [taken out of the Jepps]:
ICAO rules of the air and air traffic services
2A.1.1 (5.2.1.1) Language to be used
2A.1.1.1 Recomendation - In general, the air-ground radiotelephony radiocomunications should be conducted in the language normally used by the station on the ground.
on the other side, on 2A.1.1.2 it recommends that for universal use of aeronautical radiocomunications the english language should be used or available upon request from the aircraft station
notice should not must.
confusing ...
more to follow