antonius
Most people I have spoken to on this subject have said that Mandarin is the most difficult to learn; I have no personal idea as I speak no Chinese.
Anonymous
Sentinelese... because it's the language on that uncontacted island where the missionary was killed by the natives. It's is un documented, no one knows anything about it. But since there's around 6,500 languages currently spoken on Earth, there could be a lot of hard ones.
nineteenthly
The ones I've come across which seem hardest would be Gaidhlig, which unfortunately is kind of "my" language because I'm a Scot, Basque and Georgian.
?
I think it's Arabic language it's to difficult
heart o' gold
I'm not familiar with that many languages but I'd say Chinese is pretty difficult.