When it comes to being a financial hub English as a global language is paramount in modern world. You wouldn't expect a C-level executive from other side of the world to speak German. Further, if you've ever been in the financial scene in Frankfurt, it's unfortunately predominantly in German because most of the old people working in the institutions can only speak vacation English and they switch to German as soon possible.

Also from where did you hear that Frankfurt gets the business that's leaving London? What is your source? That was the hope for the Germans with the Brexit, but it never happened. German laws and bureaucracy is still far too hostile and outdated compared to London.