Well I'm from Finland and Finnish language really has the weirdest structure of sentences, plus there are extra alphabets. Also, we pronounce almost every single word the exact way it's spelled which seems to be a hard concept for foreigners. I've seen so many foreigners try to pronounce Finnish and form a Finnish sentence that I can confidently say, it's the hardest of them all.
I hate to say it, though someone else may already have, but English, the language in which most of us here speak, is the hardest language to learn, or so i've heard. The easiest is Spanish.. I think...
More people speak English than any other, I don't think that's the hardest, I think it would be an Oriental language, they have many more characters to remember at first, 300 in Chinese to go anywhere in the Orient, but then again, they have over 36,001 characters.
More and more people are learning Chinese. Chinese is popular, surely, it is hard to learn. I have learned it for 4 years. it only has 3500 common words. if you know 2000. you can see the newspaper now