The international airline language used to be French, until circa 1972 (I think)
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My husband was a FAA pilot examiner and flight instructor and he always told me that English was and still is the universal airline language until he died a few months ago. Complaints about pilots not having a full understanding of English has caused problems and lead to miscommunication. All his foreign students had to take their tests in English. He stopped teaching and giving exams in 1998.
https://en.wikipedia.org/wiki/Aviation_English
I think we’re confusing diplomatic language with airline language. Diplomatic language used to be French until 50s-60s I think. Airlines I think we’re always required to be English for international.