I am a UK licenced ATCO (currently "resting") and a fluent French speaker, having lived there for a number of years. I have met several French ATCO's and discussed the job and language use with them. I should stress that I have never worked in ATC in France (despite putting a good bit of effort into that aim) nor have I had the challenge of speaking my second language operationally, except on 2 occasions in emergencies.
It is worth remembering that foreign ATCO's/pilots speaking English (2nd language) have many issues to deal with. They may well be interacting with other non-native speakers from other nationalities who will all have accents - more or less heavy and more or less inteligible - before one even begins to look at cultural issues about how language itself is used. For example, my French ATCO colleagues tell me that Spanish pilots speaking English are very difficult for them to understand. An American native English accent is very different from an Australian one isn't it? So folk who point out the benefits of increased situational awareness if "tout le monde" is speaking English may ignore the possibility of a misunderstanding arising when two French people are speaking their non-native language. It should be fine at major hubs when and when standard phraseology is applicable but what about when plain speech is required for non-standard situations? In the UK level 4 ICAO English is mandated for non-natives, just as it is on the continent. I have seen foreign potential ATCO's looking for UK work who fulfilled this requirement - and had fine standard phraseology - but who would have no clue (or struggle) when talking to the fire service or ops or the bird guy, especially when the pressure was on. It all depends on the unit but level 4 worries me..... In France the standard of so-called level 4 is highly variable. No problems at the big international places but at the smaller hubs where it is less often used, one can see a much lower standard. Of ALL the guys I have met (some major airports) none had English as good as my French because it is not what might be termed "street English". In other words the kind of flexibility you get from "living the language", rather than just technical use.
To my way of thinking, this incident has some cultural issues. SIdestep being common in the US right? How much training did this ATCO have in the background to cultural expectations from the nationalities flying in there? Did she have any notion that her slip of the tongue had (probably) instinctive implications for American flyers? Could she expect to have had some training in their procedures, their phraseology etc? Would that be appropriate? Surely, their English training MUST encompass some of these points?
On a technical ATC level, I am surprised that in so many of the incidents I have looked at, the ATCO's do not update the situational awareness of the pilots, preferring to content themselves with "just" standard phraseology. There was a classic some years ago at CDG held up as a perfect example of why English only should be mandated. The lander on the outside rwy was told to hold short of the inner - which they missed - and then a takeoff clearance was given in French for a departure on that inside rwy. The departure had to abort as the arrival infringed the runway. In my view - while it is axiomatic to say that if one common language was in use it MIGHT have helped - the problem could have been avoided if the arriving crew had been given situational awareness in full; "Hold short runwy 09L -DEPARTING TRAFFIC". (Or some such.) This incident was therefore not an example of "language" issues per se but weak, non-defensive controlling. Just 2 extra words.......
PS: Gary Brown: "Understand cleared to land..." Depends on the intonation, right? A heavy upwards inflection implies a question to be confirmed. At my last place there was one pilot from the UK who regularly used "understand" as confirmation in his readback. As a native English speaker I can instinctively understand the difference.