I think the above (then looked-up a book) but the order;
Chinese 700 million
English 395 million
Russian 250 million
Spanish 230 million
Hindustani 220 million
French comes in at number twelve spoken by 95 million.
I agree definitely Chinese (Mandarin), English and Spanish; but I would guess Hindi and Arabic rather than French and Russian. Especially with languages like English, Fench and Arabic there is the complication of the number of native speakers (mother tongue) vs. the number of competent users (or people who can understand it) including as a second language.
The Cambridge Encyclopaedia of Language (by David Crystal) 1989 page 283 says that the top 20 languages in terms of first-language speakers are (n millions):
1. Chinese 1000 2. English 350 3. Spanish 250 4. Hindi 200 5. Arabic 150 6. Bengali 150 7. Russian 150 8. Portuguese 135 9. Japanese 120 10. German 100 11. French 70 12. Panjabi 70 13. Javanese 65 14. Bihari 65 15. Italian 60 16. Korean 60 17. Telugu 55 18. Tamil 50 19. Marathi 50 20. Vietnamese 50
but that the populations of the countries in which each language has official status are:
1. English 1400 2. Chinese 1000 3. Hindi 700 4. Spanish 280 5. Russian 270 6. French 220 7. Arabic 170 8. Portuguse 160 9. Malay 160 10. Bengali 150 11. Japanese 120 12. German 100 13. Urdu 85 14. Italian 60 15. Korean 60 16. Vietnamese 60 17. Persian 55 18. Tagalog 50 19. Thai 50 20. Turkish 50
although the figures above seem to be out-of-date regarding ther populations of China, India and Pakistan etc. (and in counting the USSR as one country).