Probably Chinese and Arabic, but many people say Hungarian is also very hard to learn.

Thank you,Heeyoung.Actually, I think Chinese is the most difficult language.Because you not only need to learn the pronunciation but also need to remember the Chinese characters.

I've tried learning Japanese and French, and both were difficult in different ways.

French: pronunciation, especially the "r" sound!!

Japanese: kanji (the chinese characters) because I couldn't memorize them..

Thankfully, knowing Korean and English helped me with grammar ;)