@JWD: "more native German speakers in the world than there are French" Really? Are you refering to the population of the two countries, in which case I agree, but is this still true when you add in Quebec, half of Belgium, Pondicherry and assorted French dependencies.
In any case, I would argue that French is a better choice than German because there are (probably) more people who speak it as a first OR second language (the Mahgreb, much of West Africa, Lebanon). But on that basis Spanish or Chinese are even more useful.
Of course, it depends on what you want to learn the language for. To charm the ladieeez with the language of lurve: pick French rather than German (or Dutch). To set up a sushi restaurant, learn Japanese (although most of the Japanese restaurants in the Netherlands are run by Chinese so I get a laugh with my very basic Cantonese).
In 20+ years working around the world I have picked a little of a lot of languages. There is no doubt that English is the most useful language, in all that time I have only been unable to communicate on a few occasions. Learning a few words wherever you go is always a good move (hello, tea, coffee, beer, please, thankyou and toilet is my basic vocabulary). Any language with a non-Roman script (e.g. Arabic, Thai) is hard for me but the most difficult language that I have encountered is Chinese because it is tonal, that is tone changes meaning, and I find it very difficult to hear the tone being used. Couple this with a non-roman script and ..... aargh. Oddly, my Chinese friends tell me that I am quite good at producing tones.
For those considering learning Chinese go for Mandarin rather than any of the dialects. Because I was being posted to Guangzhou (i.e. Canton) I was advised to learn Cantonese, which was not the best choice in practice. However, bear in mind that not everyone in China speaks Mandarin, many less educated people only speak their native dialect. So, if for example your intention is to work with farmers in Xinjiang be prepared to learn a dialect as well. The good news is that Mandarin, with only four tones, is one of the easiest dialects.