I would guess English because I come across far more people from non-English speaking countries that speak English than English speakers that speak another language.

Corrected to account for poor maths My instinctive answer was english and this appears to be the case. It is spoken as a second language by around 600 million people and by 300 and 380 million as a first language. This is in comparison to chinese which is spoken as a first language by an estimated 1.3 billion people. French appears to be the second most popular second language. Interestingly while french is only spoken by around 100million as a first language over 500 million have taken the time to learn it as a second language. Hindustani seems to come up on the rear of french with more native speakers (541 million) but less second language speakers with 'only' 363 million making the effort to learn it later in life. While spanish is very popular as a second language (100 milion)it is actually spoken more than english as a first language with over 400 million speaking it as their native tongue. sources: wikepedia.com http://en.wikipedia.org/wiki/Chinese_language http://en.wikipedia.org/wiki/English_language http://en.wikipedia.org/wiki/Spanish_language http://en.wikipedia.org/wiki/Hindustani_language soyouwannaknow.com http://www.soyouwanna.com/site/toptens/languages/languages3.html