PwC Experienced Associate, Advisory
Good question. You just so happen to be in luck as the main language of international business is actually English. That withstanding, knowing more than one language never hurts and it puts you in an advantageous position if you speak more than just English. I would recommend learning Chinese (Mandarin, not Cantonese) as not only is it the world's most spoken language (over 1.4 billion active speakers), but China is becoming increasingly central to global business and finance. You will make a better impression during a business meeting in China if you are able to speak the language. I learned Chinese and by no means was it easy, but it was worth it.
I hope this helps and I wish you the best of luck!!