First, the British Empire (shameful, but widespread nevertheless) introduced English to 1/3 of the world. Second, the American Cultural Empire (think Hollywood, McDonalds) has popularised (sensationalised) the English language. China's recent dominance in the international media (Olympics, next Global Super-Power) has popularised Mandarin and Cantonese in many countries more than before.

Computers dominate the world now, and computer language is in english.

I totally agree with Lee. But I would add the economic power of the US as Bluesky said. Nowadays the US is the centre of the economic activity of many countries. For example, Mexico exports up to 90% of its produce to the US, and the same is true for most countries in Latin America. China will become the next Super-power, however, I don't think Chinese would become as widespread as English, simply because learning Chinese (both writing it and speaking it) is enormously more difficult than English.

In my opinion, the world trade and economy is predominantly run by the English language and that is how it gained the world language status.