When is an accent a dialect?
It's not just languages that die off, but dialects too. Linguists tend to think of all varieties of a language as dialects. The test for whether a particular dialect constitutes a separate language or not is usually mutual intelligibility.
Where does this leave accents? These are an informal way of talking about the most obvious aspect of a dialect: the way its sounds differ from the ones we're used to.
This article explores some of the most endangered accents of English. http://www.theguardian.com/commentisfree/2014/may/27/endangered-accents-english-language