In spite of the fact that there are more than 7000 different languages spoken in the world it has been estimated that if you know 9 of them you can communicate with 90 per cent of the world´s population in everyday situations. These languages are Chinese, Hindi, Arabic, Spanish, Russian, Urdu, French, Japanese and English.

http://tukopamoja.wordpress.com/2012/01/23/9-languages-for-90-of-the-worlds-population/

According to Dutch sociologist Abram de Swaan, a sociological classification of languages based on large scale social role for their speakers :

• Supercentral languages: very widely spoken languages that serve as connectors between speakers of central languages; according to de Swaan, there are twelve of these: Arabic, Chinese, English, French, German, Hindi, Japanese, Malay, Portuguese, Russian, Spanish and Swahili;
• Central languages: widely spoken languages
• Peripheral languages: the rest – languages that not many people consider worth learning except to improve one´s own communication faculties.

http://en.wikipedia.org/wiki/Global_language_system