European Commission President Jean-Claude Juncker has told a conference in Italy on the EU that "English is losing importance in Europe".
Amid tensions with the UK over looming Brexit negotiations, he said he was delivering his speech in French.
"Slowly but surely English is losing importance in Europe and also because France has an election," he said, explaining his choice of language.
He called the UK decision to leave the EU "a tragedy".
Laughter and applause greeted his comment about the English language, and he could be seen smiling wryly.
"We will negotiate fairly with our British friends, but let's not forget that it is not the EU that is abandoning the UK - it is the UK that's abandoning the EU, and that makes a difference," he said.
UK Prime Minister Theresa May has accused some EU officials of trying to influence the UK's 8 June parliamentary election.
English v French
- English is by far the most widely spoken foreign language in the EU. There are two other "working languages" in EU institutions - French and German
- The EU has 24 official languages - it employs about 4,300 translators and 800 interpreters
- Before the UK joined in 1973, French was the main language of EU business
- Globally as many as 400 million people have mother-tongue English - the figure for French is about 220 million
The Commission's chief Brexit negotiator, French ex-commissioner Michel Barnier, has given a speech on Brexit in which he stressed the UK must agree to "crystal-clear guarantees" to protect the rights of EU citizens.
He was speaking at the same EU conference in Italy on Friday - called State of the Union, at Florence's European University Institute (EUI).
EU leaders say the UK cannot simply "cherry-pick" membership terms that it wishes to keep. A Brexit deal cannot be better than full EU membership, they stress.
France holds the decisive second round of its presidential election on Sunday. Top EU officials say they want liberal independent Emmanuel Macron to beat his nationalist rival, Marine Le Pen.
That difficult dinner
The atmosphere between Mrs May's government and the Commission soured after Germany's Frankfurter Allgemeine Zeitung published an account of her dinner with Mr Juncker in London last week.
Mr Juncker reportedly said he was leaving "10 times more sceptical than I was before". Mrs May hit back later, calling the reported remarks "Brussels gossip".
Mrs May has made it clear that she does not want leaks about the Brexit negotiations, but many doubt that such secrecy is achievable when Brexit issues affect the interests of 27 other EU countries.
Among the many potential sticking points are the size of the UK's departure bill. UK Brexit Secretary David Davis accused the European Commission of "trying to bully the British people".
A "line was crossed", he said, when stories suggested the UK could face a €100bn (£85bn; $110bn) bill from the EU.
European Council President Donald Tusk has warned that "the stakes are too high to let our emotions get out of hand".
The negotiations are expected to start soon after the UK's 8 June election.
The EU wants to sort out the UK's bill and other key aspects of Brexit before any talks on future relations but the UK government is anxious to start negotiating trade terms.
Mrs May hopes the snap election will boost the Conservatives' mandate for Brexit by increasing their parliamentary majority.
In his speech, Mr Juncker stressed that "Europe is more than just money, just a market" - distancing himself from the Eurosceptic view of the EU as little more than a trade bloc.
"There's not enough solidarity in Europe... Italy from the first day of the migration crisis has done everything it can - it is saving the honour of Europe," he said, drawing strong applause from the Florence audience.
- 7 May - French voters decide between Emmanuel Macron and Marine Le Pen as their next president
- 8 June - UK parliamentary election - Brexit talks to start soon after the vote
- 24 September - German parliamentary election, with Mrs Merkel seeking a fourth term
- 29 March 2019 - Deadline for ending talks on UK exit terms (any extension requires agreement of all member states)
- May or June 2019 - European Parliament election (without UK)
- Ratification - Any Brexit deal requires approval by European Council majority (20 out of 27 states) and by European Parliament; a trade deal requires EU-wide parliamentary approval