Generally speaking, people who speak endangered languages also speak the majority language—otherwise it wouldn’t be endangered. Preservation of endangered languages involves raising children bilingually in the majority and endangered language. Being bilingual has been linked with a lot of benefits, and the only downside is that it slightly slows initial language acquisition (but children quickly catch up).
Generally speaking, endangered languages are from a cultural minority and members of that minority culture enjoy being able to speak that language. I went on a date with an Australian man who was half belgian half japanese and he said that he wishes his parents had taught him either language, instead he just speaks english.
Our local (australian) Indigenous language is called Noongar and is undergoing revitalisation. By all accounts, the Noongar people benefit from this socially, and the language itself has information and culture.
Imagine if english went extinct. In a sense, we’d lose Chaucer, Shakespeare, Austen, Steinbeck. They can be translated into other languages (and, well, Chaucer has to be translated to be understood). But, to my mind, something valuable is lost if culture is lost. These “endangered” languages had culture too—songs and stories, maybe books and plays. That’s important.
I get the feeling that a lot of people in the rationalsphere think that if something won’t help us invent friendly AI or space travel it’s pointless. Culture’s important. Lesswrong has culture (HPMOR, the sequences, etc).
I was talking specifically about childhood language acquisition, where learning a new language doesn’t require you to forgo reading tvtropes or watching buffy the vampire slayer, it’s just part of your background acquisition the same way that children learn how gravity works and how to manipulate small objects as they grow up.
There’s plenty of research showing that bilingual children have some small advantages, e.g.: https://www.canr.msu.edu/news/advantages_of_a_bilingual_brain
Then there’s the cultural value of language that I raised in my previous post, especially for minority cultures (and you state that things from your culture like Buffy and TVTropes are valuable to you). I’m assuming you’re from an English-dominant culture. Can you imagine if you moved to, say, Portugal, and you learned Portugese and all your friends and family spoke Portugese all the time, you might feel as though something was lacking if they watched Buffy episodes that had been dubbed into Portugese?