My preferred method of choosing which language to learn is based on its economic utility. As I do tend to be a bit of the globe-trotting type, I've never really limited myself to any region or the like. Without such limitations, it makes a lot of sense to choose the languages you learn based on the percentage of world GDP represented by speakers of those languages.
And that was roughly how I chose the languages I've studied: Chinese, Japanese, Spanish, German, French, Portuguese, and Italian. I knew which countries had the largest economies and I put their languages on a checklist, so it's hardly a coincidence that the languages I speak coincide with those at the top of the top-20 list below.
|Rank||Language||Percentage of world GDP|
This list was put together by Unicode.org and the data covered by the list runs from 1975-2002, although projections through 2010 for the most part retain the same order. One interesting thing to note about the projections is that the "other" group declines to only 10%, meaning the relative importance of the top 18 languages increases. It's also worth noting, if unsurprisingly, that internet use by language largely corresponds with this list, according to data collected on Wikipedia.
There are two things that this data leaves me wanting. First is the obvious update of the data to cover through 2008, as well as longer projections going forward. Second, I'd love to see a chart based on the actual number of speakers of a all languages, rather than on the number of native speakers. For example, estimates for the number of English speakers vary from around half a billion to a billion, depending on the skill level at which you count someone as a "speaker". This would result in counting multilingual people multiple times, which might make it trickier to slice and dice the data to get a GDP figure, but I'm sure some enterprising statistician somewhere could get something we could work with.
Given the large percentage of GDP controlled by English speakers, it seems to be quite a rational choice that, if you don't already speak English, you learn it. I imagine that English's role as a global lingua franca would push it even higher if that enterprising statistician I mentioned above came along to give us the data.
I came across one memorable instance of seeing English function as the global lingua franca while interning during college in the Japanese Diet (Japan's legislature) in the office of Yuriko Koike, whose background includes a degree from Egypt and having literally written the book on speaking Arabic in Japanese. During one visit with some Arab dignitaries of some sort, they lamented that neither side was learning the other's language, but rather using English as a medium for communication. They suggested increasing learning on both sides, but somehow I doubt much ever came of it.