I'd have to say Spanish. Its spoken to a good degree of the population of 23 countries and counting, and is most likely going ot be the language taken is high school/middle school/whenever for many a students.
Plus, its easier to write then English.
Also, to point out something about the English language. I'm American, so I'd like to think that i have a good concept of speaking English. And the problem with it being the "one" language, is that its really hard to learn. First, there's two types of "English" (and yeah, I get that its just a lot of renaming and stuff, but if you asked a good portion of Americans what a Watercloset was, you're more then likely to get a blank dead stare), and that can get confusing terminology wise. Also (and more importantly), English is a hard language to write (grammatically that is).
Think of all the rules. I before E, except after C, except in words rhyming with Day, such as weigh neighbor.
Accept/Except
Effect/Affect
Well/good
The silent K and B
The sometimes sixth vowel

Plus sentence structure: Verbs, Adverb, Noun, Adjectives, etc.
When to use periods, colons, semi colons
Quotes (and how to end them)
Citation (and how to do it right, whether by MLA, APA, or other forms of paper structure).
I've seen someone teach an ACTUAL english class (where they teach to foreign individuals), and its insane how hard they have to work. The Language department pretty much all agreed (out of some... 14 languages i think, including Russian, Sanskit, mandarin, French, Spanish, Swahili, and a few other odd number choices of languages) that english is the hardest language to learn, especially if you're trying ot learn it as a second language.
Besides all that, English is the Language of business, but its losing gain there as the US dollar sinks lower and lower in its value.