Esperanto is okay, but a bit heavily biased towards european languages to claim to be "international" (IMO, anyway.) For a more unbiased constructed language, try Lojban. The only downside with Lojban though, is that learning it isn't as intuitive as learning Esperanto. But at least once you have the rules down, it doesn't get any harder to make sentences longer.

Japanese is pretty simple too, at least until you want to read or write Kanji. It also has one of the most retarded counting systems in the world, where the way to say a number changes depending on what you're counting. And also, it has so many homophones your ears will start to bleed. But apart from those minor problems it's pretty easy.