Whether the UK is part of the EU or not is irrelevant to using english as a mean of communication between europeans. I remember a study from the French ministry of education which estimated for each language, what was the percentage of the EU population, to which it is not a native language, that studied it as a foreign language either in high school or university.
German and italian are in the 15-20% range, french and spanish in the 30% area, english north of 90%.
When you have 27 different EU nationals in a room, there is just one language they can practically speak among themselves. The EU will not go anywhere if its countries resist adopting english.
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