Foxy Loxy:
I note from your profile that you are an ATCO. Can I please ask you what experience you have of talking to other ATC units on a global basis from the cockpit of an aeroplane?
I managed 46 years.
Things have improved enormously since I started. A lot of readers out there will remember the old days before SSB when we were required to monitor HF on AM throughout the flight (no SELCAL then) and also the difficulties of communicating and passing position reports when the whole of Africa and the Indian Sub-Continent were all using the same frequency.
I can well remember those days. I was faced with the problem when I was 22 years old and the only solution was (is) "to get your ear in". I could take you to a hundred places where you simply would not have a clue for the first hour or two.
I don't really want to get into the MPL debate per se; if they are well and properly trained, then they will be fine. I have spent a lot of my time teaching in a multi-lingual environment and no one nation or licence group has the perfect answer to the ATC problem.
The only secret to communicating is no secret at all. It is called "getting used to it" and my experience is that the licence held has b*gger all to do with it.
P.S. I do hope you can speak Cornish.