Next highest is generically "Chinese", with a bit less than 3m speakers. FYI...Spanish is generally a lot easier to learn for the average American than Chinese. YMMV.
If you're an American who is looking for a language to further his Engineering career, then user:informatimago has excellent observations, to which I'd offer very minor corrections:
- There is no "Chinese language". There are lots of Chinese languages. Mandarin is alledged to be universal, but even the various Mandarin dialects aren't always mutually intelligible, especially to a non-native speaker. And in the South (HK area), where much of the high-tech industry is, Cantonese, Hakka and some other dialects are spoken in addition to Mandarin. I can tell you Cantonese and Mandarin aren't mutually easy to understand. Did I mention how hard most Americans find learning Chinese of any sort?
- If you're working the refineries and oil biz, especially around the Gulf of Mexico, French is actually a good choice. Lots of Louisiana Cajuns and Vietnamese immigrants in that biz, and you get a lot of patois French creep into the conversation. Knowing some half-assed high-school French gave me an in with the Vietnamese in outside of work things (like food and such).
- Hindi is potentially good for outsourcing work as well as budget space flight. But Hindi has sort of the same issues as "Chinese"; Hindi is but one of many "Indian" languages and members of your outsource group might be native Punjabi or Gujarati speakers.
- If you're into industrial manufacturing, materials or armaments, German is hard to beat.
- There's still 37m Spanish speakers in your back yard. Unless you're career is going to make you a world traveler where all these other languages make sense, odds are there are going to be Spanish speakers above, below and equal to you wherever you work. That doesn't even count Central and South America and, or course, Spain.
But to save anyone else the hassle of replying, lots of products are made in China, you should be able to work out the rest.