From a Paris Review interview with Sarah Thomason, an expert on what happens when different languages collide:
It’s not as if people come into contact and one crowd says, Boy, your language is a lot more efficient than ours! It depends on who’s got the power. The world I live in, the world you live in, Western Europe, the United States, highly industrialized countries, the paradigm we’re used to is colonialism—and then the indigenous languages are threatened. A lot of them have disappeared and the ones that haven’t are at great risk, so that seems like the norm.But imagine a society—and again, these are mostly hunter-gatherer societies, but there are still a lot of those around—where the people practice exogamy, meaning you have to find a marriage partner outside your own group. Often the criterion is whether they speak the same language as you. If you have a society like that, you’re in contact with at least one other group and typically several relatively small groups—and it’s greatly to your advantage to maintain different languages, right? You don’t want to change your whole culture, you value your culture, exogamy seems like the way the world ought to be, and you certainly want to get married and you have this view that you shouldn’t marry your sister—then you preserve the languages.That’s one reason languages get preserved. You find another phenomenon—it’s particularly common in and around Papua New Guinea, where there are about a thousand languages. That means that they’re close together, they’re small groups. Some of them are related to one another, so they’re pretty similar, and in that part of the world it’s probably not accidental that there are so many languages in such a relatively small area. It’s fairly common for groups to deliberately change their languages so they’re not so much like the guys next door. And the most spectacular examples are where you’ve got dialects of the same language and oh, we don’t want to be too much like those guys. It’s an identity-preserving thing, it’s a distancing phenomenon.
I told you about the distancing changes in New Guinea. There’s an island called Bougainville—which is famous if you’ve read a lot about World War II—but it’s a big island and it has a language called Buin. Buin has several dialects, and one of them is Uisai. There are about fifteen thousand Buin speakers in all, and maybe fifteen hundred Uisai speakers. And Buin has, including all its dialects, a very elaborate gender system, sort of like what you find in French or Russian or German but more elaborate because each noun is either masculine or feminine, and then the verb will agree in gender with the noun, and the adjective will agree in gender with the noun, and so on. So in a sentence you’ve got a lot of markers indicating the gender—it’s part of the syntax as well as the lexicon. But in Uisai, all the genders are reversed. Every noun that’s feminine in Uisai is masculine in all the other dialects of Buin.Now, this just isn’t conceivable as any kind of ordinary, natural, gradual linguistic change. I mean they have to have sat down and said, We’re too much like those guys, we’ve got to do something. How about this? A lot of linguists, maybe most linguists, would say this isn’t even a possible linguistic change. My belief, which has gotten more radical the older I get—which is nice, you don’t want to get intellectually fossilized—is that anything you can become aware of in your language, you can change if you’ve got a powerful enough motive. And of course, it’s not going to affect anybody’s language but yours, unless everybody else changes, too.