As a (former) Japanese major, I have to stick up for my language here. Japanese is only spoken in Japan, yes, but Japan is a reasonably influential and important country, particularly in science and business. It may not be as valuable as Chinese and Arabic in terms of employability, but it certainly does increase your employability; I'm pretty sure it was one of the major factors that got me into graduate school. Furthermore, in terms of difficulty, it is Serious Times due to the relative incompatibility of many grammatical concepts with English ones. Passive/causative/passive-causative verb forms, collective suffixes, on- and kun- readings, and politeness levels make gendered nouns and nominative/accusative/dative look like the Junior Jumble. I may be overexaggerating the case due to the infamous sadism of my college's Japanese department, but seriously, if you want a pure linguistic challenge, it's up there.
Anyway, Chinese (or one form of it, anyway) and Arabic are also pretty solid choices. Honestly, if you learn any language with a quintuple-digit-or-better native speaker population other than the non-Spanish Romance languages and possibly German, you will probably have a serious professional edge for it, and I'm sure I don't have to convince anyone gay for Latin about language learning's beneficial effect on your life. Rock on.