Contrary to popular beliefs, Japanese is easy. The tricky part is reading and writing but that's because of all the Kanjis, the third Japanese writing system. Arabic and Russian aren't hard either, but one has to practice them daily before mastering them. I'm Lebanese so Arabic is one of my native languages (Alongside French and English) although I stopped Arabic because I wanted to learn Spanish instead. Anyway, we shouldn't count the minor languages because they are too hard for anyone to learn. You have to grow into them or else you can't understand a word they're saying, like the Inuits for example. Out of the famous languages, I think Thai is the most complicated language to learn. It has a lot of similar sounds (About 60 ways to say Cai I think). Traditional Chinese is very very hard as well but not impossible, many people are learning Chinese (and Japanese) nowadays. Russian isn't easy but it isn't very hard either. My Tennis Teacher learned Russian on his own (his wife is Russian).
I guess it's all relative. I mean Spanish is harder for an American or a Russian than it is for a French or Italian. It's been a year since I started Spanish and I can already understand most of the time, I still have trouble creating sentences but just another year would do it. Similarly, Japanese is hard for a westerner than it is for a Chinese. Not because they are similar (except for the Kanjis, which litteraly mean Chinese Characters) but because they have more similarities than with Western languages.
I speak and write 3 languages (Arabic, French and English). I understand Spanish and little of Japanese, I'm currently learning both languages.
I plan on learning Mandarin Chinese, Italian, Sign Languages and maybe Latin as well (I love languages), which me good luck.
Joey Ayoub