Most difficult languages are Navajo, Finnish and Hungarian.
Interestingly they all originated from the Altai mountains of central Asia.
I speak Hungarian. It was unbelievably difficult to learn. It has certain structure/concepts I had never seen nor heard of in any other language - just wild. That is all in addition to the vocabulary being completely different. You can look at several billboards when you go to Hungary and not understand one single word. That will not be the case if you go to a country that speaks a Germanic or Romance language.