The accuracy and impartiality of that Wikipedia list is disputed, it says to right at the top of the page. The top ten list in the Washington Post in 2002, for ranks in 1998, is significantly different:
Mandarin.... 874,000,000
Hindi.......... 366,000,000
English....... 341,000,000
Spanish...... 322,000,000
Bengali........ 207,000,000
Arabic......... 201,000,000
Portuguese.. 176,000,000
Russian....... 167,000,000
Japanese..... 125,000,000
German....... 100,000,000
"Language follows the coin, not the flag," is an old proverb among linguists. America is the economic giant today so English is the world's leading language. We should remind ourselves that French had its day in the West, as did Latin and Greek. Aramaic was the leading language in the Middle East for centuries, and we can hardly forget that Chinese monks spread their language throughout East Asia with their writing system and other culture.
Perhaps Chinese will have another period of ascendancy. Personally I think it's a much more suitable language for a rapidly changing world. It is far more adaptable than English, which is hobbled by Stone Age paradigms like tense, number, and parts of speech. We have to express relationships with a clumsy set of less than two dozen prepositions; Chinese does it with nouns and verbs.
Americans complain about the "difficult" phonetics of Chinese, but most of the foreigners I've met think that English phonetics are positively daunting. Chinese has the unique advantage, due to its near total lack of noise-words, of being compact. It generally takes fewer syllables in Chinese to express an idea than in most other languages. As a result it can be spoken more slowly, a boon for non-native speakers struggling to parse sentences in real time.
My vote, if it were up for election, would be Chinese. My prediction, based on the direction of the world economy, is also Chinese. Of course I'd be happy if it were Esperanto because I already speak it, but to me its intractable disadvantage is far too many syllables.
