Before answering, I have to study all the existent languages.Then, it would be possible to compare :). Just kidding. Now, being serious. In general, the farther the second language is in relation to vocabulary, sounds, sentence structure, culture, and other factors to the learner's native tongue and culture, the more difficult the acquisition will be.

klingon

for me is French , because i found it very difficult ( to pronounce some words correctly ) but i challenge myself to learn to no matter what :)

French - It feels like I'm going to bite my tongue when I try to speak in French. ^_^; Sinhala - The alphabets! OMG! A lot! Still not memorize all those cute curly letters yet. And the keyboard layout, too, makes me so confused when I have to switch between English and Sinhala. And the worse of it, I can not find Sinhala dictionary anywhere in Indonesia (and not so much sources on the net, too). >.<

sinhala