The maintenance of Yiddish is an extreme example. It's explicitly used to prevent ultra-orthodox Jews from "straying off the path" and joining the larger society.[1] In Israel, the ultra-orthodox speak Yiddish, not Hebrew, and often don't even know Hebrew.

This is the use of a dying tongue as a means of social control.

[1] http://www.newyorker.com/news/news-desk/off-the-path-of-orth...

I think it's a bit different - these communities (which BTW are a minority even among orthodox religious Jews) choose to speak Yiddish and not Hebrew to express their detachment and disconnection with modern Jewry and specifically Israel. Secondary consideration is the status of Hebrew as the holy language, not to be used in mundane and profane matters. This was long the case in the Jewish diaspora, but with the start of the Haskalah project and the idea of Israel as the Jewish state, this has changed. These communities have not accepted these changes and thus try to preserve their way of life as it was beforehand. The use of Yiddish here is more of a consequence than the reason.

That's an interesting take on it. I wonder if there's much history of groups starting out with common languages, then groups separating and the languages evolving separately, either intentionally to demonstrate group membership, or just due to lack of communication between groups, and continuing to differentiate until they're effectively different languages.

I only speak English, but I've heard several different accents on it so strong that they're hard to understand. I wonder if the speakers of these accents find it impossible to understand other strong accents.

Well there you have it. The vocabulary teens use varies from city to city. The vocabularies of (France) French and Canadian French are different to an extent, as are those of (England) English and American English. It seems to me completely obvious that "one language to rule them all" is an absurd idea bound to fail. It will morph and splinter by itself.

My mother-in-law is from an island off the coast of Honduras. She's a native English speaker, but has a strong accent, and the dialect of English that they speak has grammatical differences, along with some different vocabulary. I suspect that some comes from Spanish and some from Garifuna.

Without modern communications, their dialect of English would've continued diverging, and eventually be considered different enough to be a new language, rather than just a dialect.

much history? This is exactly the way new languages evolve.

> I wonder if there's much history of groups starting out with common languages, then groups separating and the languages evolving separately, either intentionally to demonstrate group membership, or just due to lack of communication between groups, and continuing to differentiate until they're effectively different languages.

Thats basically what sociolinguists investigate. Here's an example of a study that kind of does that: https://www.researchgate.net/publication/233713880_A_Quantit...