I would decide first where you want to go, then pick the language that will help the most. Though the language to learn may be a part of the decision on where to go (Learning French will obviously be easier than any other language, and would help a lot if you want to travel Africa and to a lesser extent Europe).

I agree that people who want to communicate invariably will. I knew nothing of Spanish before leaving on my trip (now in Peru). I took a 2-week immersion course in Zacatecas, and listened to the audio series by Michel Thomas (which are fantastic, especially if you are an analytical type). But for the past six months or so I've not studied much. My Spanish continually improves. The biggest challenge is just getting used to the different accents as I move around, and convincing people to talk slowly. It has been slow going and I fully admit I could improve a LOT faster if I studied more often or took more classes. But I can have surprisingly complex conversations even so, and can trivially get by on daily stuff.

All that said, I wish I had learned a bit of the language before I left. It would have helped enormously. Or perhaps taken a month of immersion lessons instead of just two weeks.

And don't discount reading those books. Vocabulary is by far the hardest part of a language (for me, at least), and reading is a great way to get more vocabulary and an instinct for correct grammar. Pronunciation can change so much from one place to the next that you may need to frequently relearn that anyways....

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Traveling The World Since: 2011
Blog: The Seductive Life (General Travel)