AHC:75% of New Guinea speaks the same language

With a POD after 1750, have 75% of the population of New Guinea speak the same language by 2000.

The combination of high, rugged mountains and impenetrable jungle make New Guinea one of the most difficult places in the world to unite in any way at any point in history. My first instinct here would be to have the entire island come under the dominion of a colonized that for some reason becomes quite serious about importing colonists, and have them spread across all the coasts, using modern high-intensity agriculture and medicine to sustain a population so high that it's at least 3 times larger than the population of natives. They could, for example, clearcut and settle on the large plain on the southern portion of the island.

Say there is a great famine or disease on the island sometimes during the 1700s that wipes out much of the population. Then, later on, New Guinea is settled by the English/Dutch what have you. Remaining natives are killed or assimilate and you may get a New Guinea that is 75% English or Dutch speaking.

You actually need a PoD before 1750 for an indigenous New Guinea language to dominate the rest.

Or, more and more people across New Guinea adopted a variant of Malay as their mother tongue.

Actually Tok Pisin may well be spoken (to some degree or other) by 75% of the population nowadays.

Give it another 100 years.

Primary or second language

It took the Philippines less than 50 years from a Spanish/native speaking society into an English speaking society from 1901 until 1946. From anti-Americans to pro-Americans. PUblic education+political unity will do wonders within one generation.

USA has proven that it can be done in otl.