The section of the US Government responsible for RT is the Federal Communications Commission (FCC), not the FAA. I was amused to see my German RT licence annotated 'Issued on the basis of FAA licence number...' which was total BS.
There I had to do a rather thorough test in order to use German, done with the Ministry for Post and Telecommunications. That involved a run-through of an imaginary CVFR flight from Hannover to Bremen, from calling for start-up to parking on the ramp at destination. For an American RT licence I just had to send in a little postcard which came back stamped. Full marks to the Germans on this one.
I just did the British RT test, again with an imaginary flight. Of the two the German test was more thorough but the British one would suffice, I guess.
There was a high-profile crash at Kennedy a few years ago when a crew from South America ran out of fuel. The report had the Captain, with no English, unable to get the FO to call a Mayday, so that they were given normal vectors instead of priority handling. That one was not strictly down to lack of English but it must have been a major factor.
The running joke down in south Florida was the guys up at Opa-Locka, trainees from the Americas. Every so often the tower would instruct one to hold short when he would just reply, 'Royer!' and give it the berries. I found one wandering around, terminally bewildered, on short finals for 09L at Miami International when he must have been looking at Krome Avenue and the vastness of the Glades without looking the other way to see a rather large international airport!
Nigeria, a few years ago, had to tell a few guys to go back home to eastern Europe when it seemed that their English was too scanty, so it's not just a problem in the developed world.