I think the answer of you title is multiple, nubilous and countless. Different individuals will have different THE MOST DIFFICULT LANGUAGEs IN THE WORLD. The difficulty of a language depends on the learner's experience. For isizulu and isiNdebele language speakers, they can easily learn Xhosa. Because the three languages share similar grammar rules and vocabulary and the speakers live very near. There are a lot of Arabic lownwords in Persian. So Iranian can learn Arabic more easily than any non-persian and non-semitic languages speakers. The same to Chinese and Japanese. If two languages are very close (dialect continuum), then a speaker can easily master the other language. Just like Russian-Ukrainian-Belarusian, indonesian-malay and Deutsch-Dutch... In China, college students from foreign language department think Russian, Arabic and Deutsch are difficult. One of my friends said Georgian language (or any other Kartvelian language ) is the most difficult in his cognition. I don't think there is a language deserving the title "the most difficult language in the world". Language's Difficulty varies from person to person. And I think Analytic Language is the most easiest language. Anyway, good to see that Chinese is difficult. Ha ha ha...