So this is the President of France postulating that when his country takes the Presidency next year, he will use that to push French to be the next working language. I don’t know what the President of the Council of Europe does besides Chair meetings, but apparently it includes setting the language for which the meetings will be held in.
So this isn’t something the EU as a whole has determined will be policy going forward, more like the first guy to drop his pants in a newly opened frontier for dick-waving. If this was something the President of the Council of the European Union could do unilaterally anyway, then my read is that Emmanuel Macron would have pulled this stunt, Brexit or no Brexit, and Brexit is just convenient political cover.
More likely this won’t last more than the 6 months that France has the Presidency, and it will be amusing to see if say, Hungary the next time they have the Presidency insists on high level communications in Hungarian.