Cornish language
According to Wikipedia:
The Cornish language revival (Cornish: dasserghyans Kernewek, lit. “resurrection of Cornish”) is an ongoing process to revive the use of the Cornish language of Cornwall, England. The Cornish language’s disappearance began to hasten during the 13th century, but its decline began with the spread of Old English in the 5th and 6th centuries. The last reported person to have full knowledge of a traditional form of Cornish, John Davey, died in 1891. The revival movement started in the late 19th century as a result of antiquarian and academic interest in the language, which was already extinct, and also as a result of the Celtic revival movement. In 2009, UNESCO changed its classification of Cornish from “extinct” to “critically endangered,” seen as a milestone for the revival of the language.
That article links to the article about the Last speaker of the Cornish language, which starts out:
Identifying the last native speaker of the Cornish language was a subject of academic interest in the 18th and 19th centuries, and continues to be a subject of interest today. The traditional view that Dolly Pentreath (1692–1777) was the last native speaker of the language has been challenged by records of other candidates for the last native speaker, and additionally there are records of others who had knowledge of the language at a later date, while not being native speakers.
Finding the last speaker of the language is complicated by the lack of audio recordings or transcriptions owing to the date of the language extinction. It is very difficult to know, without such evidence, whether those reported as speaking Cornish into the 19th century were able to speak the language fluently, or even whether they were speaking it at all. A substratum of Cornish vocabulary persisted in Cornish English, and in some cases those identified as Cornish speakers may have been speaking English with heavy Cornish influence.
I encountered the Cornish revival in the academic book The Celtic Languages, ed. Martin Ball and Nicole Muller, which provides a bunch of articles about the Celtic languages, covering some of their history and linguistics. A lot of it went deeper into linguistics than I have the background for, but I found some of the book really interesting.