What a great skill to add! I've been working across the globe for the last decade and whilst it obviously depends what part of international business you're most interested in, I would probably lean more towards Spanish or Mandarin in business. Both are increasingly useful but if your heart lies in German, French, Japanese etc, it would certainly hold you in good stead as well! The main thing is that you're personally invested in learning the language for a long period of time to build fluency, so personal interest matters just as much as applicability to business :)
Hope this helps!