Second Language

For someone who only knows English, what does everyone think is the best supplimentary language to learn. I'm thinking German, since it's used through a lot of Europe (or at least gives one a better chance of communicating with someone from some parts of Europe) and is similar to English.

Germany's Population: 82,431,390
Japan's Population: 127,417,244

Not "much larger", and both Japan *and* Germany have 'worthwhile culture'