It's hard to believe that the Brits don't understand the concept of translators.
You've never had to rely on translation, have you?
It's like gambling at the casino. You never know if it's correct.
Eurocrats tend not to use translators. All EU business is usually conducted in English - the world's lingua franca - and not a minor, declining language like French.
Even British schools are stopping teaching French now because French, like France, is in rapid decline and not seen as that important anymore. More schools are teaching Chinese instead.
I'm sure Monsieur Barnier is just being your typical petulant Frenchman.
From my experience, the world's lingua franca is broken English, even in Quebec. You just never know whether the message actually got across to your interlocutor.