More works are translated into French than any other language (even English); so, it would be useful to, at least, learn to read French. As far as geographic distribution of speakers is concerned, Spanish, Russian and Arabic are English's main rivals. None of these are that particularly "easy" (familiar in stucture and vocabulary) for Chinese speakers, but Spanish has a reputation for being one of the easiest of the languages to learn as a second language among English speakers, even though it is, by far, not as closely related to English as at least a dozen other European languages.

The easiest one is Toki Pona (http://tokipona.org). It's polysemantic and has only 125 root words. So you can express the most, using the least.

Well, Bill and Daniel gave you some pretty good answers, with good arguments. I'm gonna advice you to learn Spanish language, but my reasons are just because I love my language, so I'm completly subjective :)

I guess it has got to do with what you do (your profession, your work). For example, if your company is doing business in Thailand, learning Thai would be useful. But in general for Chinese (China), it is easier to learn Korean as there are some similarity and is close proximity to China. Useful or not depends whether you meet any Korean or has any Korean friends.