Not going to happen without an agreed single working language, which can't realistically be anything other than English, and that'll be unacceptable, so the idea is (very sadly) dead on arrival.
For a business, the language is whatever you agree with your customers. You are free to choose. If you communicate with the EU itself, any EU country language is accepted.
Language diversity is just a fact of life in Europe and not going away soon, that's like asking the US to give up the idea of states and become a single country.
It's still going to be English, although not by a huge margin if you include non-Mandarin Chinese.