BRUSSELS—As if the European Union didn’t have enough to worry about, now it’s at odds over what should be its dominant language.

The French president, Emmanuel Macron, wants to make French grand again and replace English as the default language in EU institutions, the way it was before Britain joined the bloc in 1973. With the U.K. negotiating to leave the EU next spring, he is eager to restore the linguistic ancien regime.

“English has probably never been so present in Brussels as when we’re talking about Brexit,” Mr. Macron said in March on the Day of Francophonie—a celebration of French language and culture observed in more than 70 countries. “This domination is not inevitable,” he declared.

Dethroning the Queen’s English looks nearly impossible for the soon-to-be 27-country bloc. Its 24 official languages produce 552 translating combinations—an impractically large number that demands a shortcut.

English is by far the leading foreign language taught in the EU, according to official statistics. Over 80% of primary-school children and over 95% of secondary-school students across the bloc learn English before any other foreign language.