Because you never know when you and another person will need to converse in two different languages, there's Google Translate. And don't forget that the louder you speak in a language people don't know, the better they'll be able to understand you. That's how language works!
It's as close as we've come to a true on-the-fly translation machine. Google Translate allows you to speak English into your phone and have Spanish come out the other end, and vice versa. There are other supported languages too.
Download links: Android | iPhone
See also: iTranslate, WordLens, Transzilla Translator