1. Chinese - There are many dialects of Chinese of which the most spoken, by far, is Mandarin (about 850 million), followed by Wu (90 million), Cantonese (Yue) (70 million) and Min (70 million).
2. Hindi- Hindi is an Indo-Aryan language with about 490 million speakers who use it as a first language and up to 225 million who use it as a second language.
3. English: Approximately 341 million people speak English as a native language and a further 267 million speak it as a second language in over 104 countries.
4. Spanish- The Spanish language is spoken by more than 358 million people in Spain, the Americas, Australia, and Africa. In the early 21st century, Mexico had the greatest number of speakers (more than 85 million), followed by Colombia (more than 40 million), Argentina (more than 35 million), the United States (more than 31 million), and Spain (more than 30 million). Spanish is the official language of all these countries except the United States.
5. Arabic: In the Middle East and North Africa it is the first language that is spoken by 280 million people. 250 million speak it as a second language
Source: Knol Author Foundation