In 1978 I operated into JFK for the first time during training, BOAC had excellent briefing documentation and I had written down the expected clearance..I said say again twice and on the third go I thought I had read back the clearance correctly..at Deer Park the captain did something I didn’t expect and challenged him..I had got it wrong.
The following year I started working for the Swiss and was impressed by they linguistics until I had a rollicking from a Maastrict controller because the captain hadn’t understood me. I then drastically changed the way I spoke; annunciated in extremis, slowed my delivery, used simple words and always asked a question or relayed information that needed a response which couldn’t be answered with a yes or no. Such was the change that when I visited my old BEA crew room one of my mates thought I had a stroke.
It saved further internal cockpit confusion but we blocked a runway - probably Chicago- where we caused a missed approach..the captain never realised although the controllers instructions were clear.
Around 15 years ago I used to operate in Montpellier controlled airspace for 4 or 5 hours at a time..they had a female controller who obviously didn’t speak adequate English.
Maybe one day a system will be used which doesn’t rely on spoken clearances, writing them down and conflict warning such as on my car. The technology is there.