If your only goal is to attract economic migration then this may be fine. I'm quite happy with keeping the diversity of European languages intact and for someone who wants to make a real commitment to a country learning a language shouldn't be a problem.

Also for what it's worth I moved around Europe as a kid coming from a lower middle-class background, didn't go to international schools, and learned two languages just fine. Kids pick languages up fast.

Also what has speaking English or intra-european migration to do with fertility rates? For the European fertility rate it doesn't matter how much you move around in Europe.