the most difficult language is the one you never try to learn ;)

Russian & Finnish would be very difficult to learn for me because these languages have a variety of cases in terms of grammar. 

In addition, I would like to add German, to quote Oscar Wilde, "Life is too short to learn German."

I think it depends what the learner's native language is. For a native English speaker I agree it would be something like Arabic or Cantonese.  Possibly also Native American languages due to difficulty to access learning resources and "native" speakers (no pun intended). 

I think Chinese and Arabic are the hardest.

The vocabulary the whales use to communicate is still quite mysterious.

Scientists and daydreamers are scanning through the oceans to

hear a bilingual whale speaking. They are enduring painstaking

efforts for this ambitious goal.