This is a really intriguing question for me. When (if ) the UK leaves the EU then not 1 country will have English as their mother tongue. It would seem crazy to continue with it as the "principal" language , but really any alternative is fraught with danger. French used to be the diplomatic language for the West, but their time has passed and other than a few African nations, French holds no sway in the larger world. Germany have the biggest population in Europe, but 2 World Wars have ended any real hope/desire for Europe to speak German - can you imagine it. Spanish does have a large influnence in the world (because of those catholics popping out kids) but for the most part it is seen as a language for the world's poor and don't forget even in Spain it is not a truly accepted first language. Russian - no way, jose. Chinese would be a good choice but it isn't European so it would be unacceptable and of course the Americans would simply boycott Europe . Personally I think that Arabic WOULD be a good alternative. Due to colonialism and empires, France, Germany and Spain have an amazing number of Arabic speaking people who could be/are European nationals and it would be a language which most of the European countries have links to without any claiming - but it isn't going to happen. Racism is already too high throughout Europe.
So really it has to remain English - but how can that be justified by the EU?
Written like that it looks like Dutch!