He was speaking at a conference in Florence, Italy, amid tensions over Brexit negotiations with the UK government, the BBC reported.
Juncker said his speech was in French "because slowly but surely English is losing importance in Europe and also because France has an election".
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Both laughter and applause greeted his comment about the English language, and he could be seen smiling wryly.
The conference, on the state of the EU, comes at a time of tensions between Brussels and Britain ahead of the opening of formal negotiations over Britain's withdrawal from the 28-nation bloc.
“UK’s decision to leave the EU is a tragedy,” he said. "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."
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