Language revival refers to the process of “resurrecting” a language that was either extinct as a first spoken language, or marginalised and moribund. (See also: language loss.) By “resurrecting”, I mean there are a range of actions that could be taken: teaching a standardised form of the language in schools is a major one, but it could also refer to a range of other language promotion activities (funding TV programs or radio shows in the language, offering grants to community groups, etc.).
Language revival is generally viewed positively but the process can be contentious, sometimes. Where a language was not completely extinct, the process of standardising it in order to teach it to outsiders can be controversial, because that means an artificial form of the language is being elevated above the “authentic” dialects which are actually some people’s first language. Some “revived” languages that exemplify this issue include Manchu, Romansh, and to some extent Irish.
There are also cases, particularly in the context of indigenous languages in settler-colonial states, where speakers of a moribund language refuse to allow outsiders to document their language, let alone learn it or teach it to other outsiders. This is basically out of the concern that outsiders will not respect the cultural aspects of their language (e.g. the separation of liturgical and everyday speech) and would rather their language just die than be disrespected by colonisers. Of course, it’s not like all speakers of endangered indigenous languages feel this way. But as far as I know, it is pretty much universally true that they prioritise passing language on to young members of their own community than to rando language enthusiast outsiders.
On the flipside, the one big language revival success story that no one can deny has been Modern Standard Hebrew. For centuries leading up to the 19th, Hebrew had been used as a liturgical language as well as, sometimes, a lingua franca for Jews from different parts of the world who didn’t share any other language, but it was no longer a first language for anyone. While modern Hebrew differs from older forms of the language in a number of ways (with some grammatical and pronunciation differences as well as a huge number of newly coined or borrowed words), it is very clearly a healthy, thriving variety of Hebrew spoken by millions. Nor have I really heard of anyone (no one who speaks Hebrew, anyway), wringing their hands about any “authentic Hebrew” being usurped by this standardised form. Sure, this is perhaps because there weren’t any native speakers of so-called “authentic Hebrew” left at the time the modern form began to be promoted. Still, overall: a success story.