Mandarin is the 2nd most spoken language in the world, only behind english. Its also the 2nd most important language for programming, and has the largest open-source community in the world, so that'll be a plus for my work.

I'm also learning spanish, and am much further along there than I am in Chinese. I feel like with those 3 languages I'll never be too far from someone who can understand me.

Oh and for how easy it is. If you're coming from a germanic or a romance language, it shares almost no words and roots, which means its going to take overall its going to take longer. But in most ways its much easier and more logical. No verb conjugations, no weird plurals, easier tenses, logical word and character construction. I'd argue its even better constructed even than esperanto, which still kept the weird verb declinations.

The only extra annoyance is measure words (which english has also, ie a flock of birds, a pile of clothes).

I'm learning Mandarin right now as well, and in some ways it's much easier than other languages I've learned. Grammar is pretty straightforward, there's not conjugation or tenses to worry about either. If you learn a word, you just have to learn it once, there aren't any variations. The whole thing with tones is largely overstated I find. Even if you get the tones wrong, people will understand you from the context. Where it gets trickier is with writing because the character based system is genuinely more difficult to learn than an alphabet. The characters are basically words written in two dimensions instead of one. Most are composed of subcharacters of which there is a common set of. But the upside here is that once you learn them, reading is a lot faster because each one is basically like an icon. So, you can scan through text a lot easier than with words all written out left to right.