Great! I propose Spanish, being the second most spoken language by number of native speakers (behind Chinese), and given the relevance of Spain through history in Europe.
(this comment was only half joking. It's easy to impose a global language when it's yours, but you see, it's a different story when you need to learn a different one).
Debatable. I feel that business level fluency/proficiency is easy: limited vocabulary, limited jargon, overall formal tone.
Getting to a conversational level in any kind of social settings, now that's the hard part of any language, in my opinion.
Example: Some court proceedings in Germany can now be done in English:
http://www.fgvw.de/en/news/archive-2015/the-introduction-of-...
This said, the market is adjusting on its own. EU customers are voting with their wallets, and the more they move around, the more English-speaking cross-border businesses flourish.
Officially, France (and the Academy) only barely recognize the existence of French spoken in their former colonies, and even then only when it's convenient (to maximize the count of the number of French speakers). They go out of their way to try to "preserve" the French that is spoken by white French people in France itself as the only official version of French, so they sure as heck aren't going to recognize other languages spoken in France by minorities other than French.
https://en.wikipedia.org/wiki/Vergonha
My grandparents speak Breton too, though not with me. As children, they used to have their fingers smacked if they happened to say a word in Breton. Back then, the French of the Republic, one and indivisible, was to be heard in all schools and those who dared challenge this policy were humiliated with having to wear a clog around their necks or kneel down on a ruler under a sign that read: "It is forbidden to spit on the ground and speak Breton".[13] That's the reason why some older folks won't transmit the language to their children: it brings trouble upon yourself...
After having lived and worked in multiple EU countries, I'm convinced that language is the single biggest barrier to true freedom of movement of workers in the EU. In Germany, for example, you cannot assume any daily task can be accomplished without the German language. You're then faced with either constantly dealing with communication problems with everything in your life, or paying the significant opportunity cost of learning a new language -- which you will likely never learn to high enough degree to fluently use in business, and which won't help you in the next country in which you live.
What is needed is a joint common language (English) that everyone in the EU learns starting in kindergarten and in which you're guaranteed state services in every member state. But there is no chance the French and German speakers would ever agree to that, it is like a religious issue.