Anatoly Karlin has a good post on the top 10 most useful world languages.
For more detail look there. His ordering goes as follows, though I’m not sure I agree.
My own ordering would be something like this:
1. English, no doubt
2. French, still the language of culture and of the chic, despite its precipitous decline in importance. Most Arab businessmen prefer to use either French or English to Arabic
3. Chinese, surging in importance and use, though it’s hard to get beyond the basic stage
4. Arabic, despite the dialects making it confusing, if you can speak Arabic and you live in the west, you’re government will employ you to do something with it
5. Spanish, populous and great for world travel, but still not that important culturally or economically. An advantage is that it’s easy to learn, but that also makes it less of a skill to know
6. Russian, lots of culture, but declining in importance; if you know it, however, you’ll find a use for it
7. German, still strong, despite its similar decline and useful in business and much of culture
8. Japanese, in decline as well, but chic and a useful skill to know
9. Portuguese, growing in importance, but still not that powerful culturally; useful in travel industry
10. Turkish, becoming more important economically and politically, but a somewhat isolated tongue with little cultural output or prestige