These numbers are counting only first language speakers, so I think it's a waste of time speculating about larger colonial empires in Africa and Asia (unless you're putting settler colonies there).
1. Mandarin Chinese (all types) - OTL 1st, 955 million speakers
OTL
2. Portuguese - OTL 6th, 215 million speakers
A stronger Portuguese Empire, in detriment of the Spanish and the English. We can have the Portuguese colonzing Newfoundland, the Plata basin and Pampas, some chuncks of Australia (plantations in Queensland, for exemple), and retaining Malacca, making it a Mestiço majority city. Also an earlier (the earlier the better) effort to put settlers in Angola and Mozambique.
3. Spanish - OTL 2nd, 405 million speakers
As I've said over there, let's give the Plata basin to the Portuguese. If more is needed to bring it down more, lets make some native language is some part of the Americas stay strong to the point of Spanish being forgotten after the end of the colonization, like what happened in the Philippines.
4. Hindi - OTL 4th, 310 million speakers
OTL
5. French - OTL 18th, 75 million speakers
A stronger French colonial empire, keeping most of it's North American possessions. We can also having them establising a colony in Western Australia. You can also have France proper avoiding the XIX demographic stagnation.
6. Arabic - OTL 5th, 295 million speakers
Some more non-Arabic speaker in the Arab world. Berbers n the Magreb, Copts in Egypt, and Syriacs in the Fertile Crescent.
7. English - OTL 3rd, 360 million speakers
As I've said, let's give good chuncks of North America and Australia to the French and Portuguese. To avoid a drastic loss of English speakers because of the French controling Canada and Louisiana, let's have the British colonizing OTL's American and Canadian Pacific coast. Also, have the Irish language stay strong in Ireland.
8. Bengali - OTL 7th, 205 million speakers
Make the Koch language stronger in northern Bengal.
9. Greek - OTL 75th, 12 million speakers
A surviving Byzantine Empire, keeping Anatolia, Antioch, Sicily, Calabria, Apulia, and Crimea.
10. German - OTL 11th, 95 million speakers
A slighty larger German population. Just avoiding WWII would have been enough for this.
11. Russian - OTL 8th, 155 million speakers
Avoid the fall of the Kazan, Astrakhan, and Sibir khanates, so keeping Russia restricted to Europe. Also avoid Russian expansion over the PLC, so to not create Russian-speaking populations in Belarus and Ukraine.
12. Japanese - OTL 9th, 125 million speakers
Make the Ainu stop Japanese advance to the north, and estabilish a kingdom in Hokkaido and Tohoku.
13. Punjabi - OTL 10th, 100 million speakers
Make the Gandhari language still spoken in the Hindko area.
14. Javanese - OTL 12th, 82 million speakers
Make Sundanese be spoken in Cirebon.
15. Vietnamese - OTL 16th, 76 million speakers
A slight strong Vietnamese presence both in the Mekong delta and in the mountainous northern frontier.
16. Persian - OTL 29th, 45 million speakers
Make the OTL Azeris speak Persian.
17. Telugu - OTL 15th, 76 million speakers
A stronger Hindi-Urdu presence in Telegana.
18. Wu - OTL 13th, 80 million speakers
Shanghai isn't founded.
19. Malay - OTL 14th, 77 million speakers
A smaller (or just no) Malay colonization of Borneo.
20. Korean - OTL 17th, 76 million speakers
Some parts of northern Korea are Sinified/Manchurized.
Marathi - OTL 19th, 73 million speakers
A stronger Hindi-Urdu, or Gondi, in Vidarbha.
Tamil - OTL 20th, 70 million speakers
A stronger Sinhalese in Sri Lanka, and Sri Lankan Moors stay speaking Arabic.