Your going to find the most use out of a language where the native speakers don't already speak English. For instance, I'm fluent in Swedish, (Read, Write, Speak). English is a required class in Swedish schools from the 3rd Grade until graduation of High School. Most Swedes speak better English than I do. German is about the same thing. If your visiting a country where education and the economy is good, English will get you by just fine.
Spanish, Mandarin Chinese, Arabic, and Rusian are going to be the most commonly spoken languages where the person your going to meet might not speak English. Which ones you choose to learn would depend on where your likely to travel.
I will say that Spanish will be more useful when your in the States than any other "second" language. I'm currently working on learning Spanish, and I don't really plan on global travel any time soon.