If anyone is interested in learning the Old Celtic language Manx Gaelic, I have created flashcards on Duolingo Tinycards. I will be making more Manx flashcards and other languages such as Irish, Scottish Gaelic, Breton, Cornish and Welsh! I'm following everyone who follows me :) Here's my profile if you want to have a look at the cards https://tiny.cards/users/WelshPolyglot246
In the future when I'm much better at Manx, I might even help create an actual course here on Duolingo but I will need help from other Manx speakers :)
Good luck to you with your "reviving dead or dying Celtic languages" :) I'm sort of crazy about the idea of the revival of the Celtic languages ever since I started the Irish course here. I'm still to give Welsh a try (I'd like to very much but first I need to do a few other courses that feel 'more urgent' to me). Would love to see other Celtic languages on Duolingo though.
From what I understand Manx is closer to Irish than for example Welsh. It doesn't seem very familiar though, unlike Scottish Gaelic. Due to the different orthography, perhaps? How are they similar? Good luck with this and with the other languages once again :) I hope someday when I get to learn Irish or the other Celtic languages well, I'll be able to do something for those revivals myself.
People often say half-jokingly that in order to learn Manx, you just have to write Irish with English spelling rules. I haven't looked much into Manx, but from what I saw I would certainly agree that the orthography contributes much to your impression. When I tried, I had a hard time guessing most words, but once I was told the English translation, I could often see the Irish cognate (though sometimes it is just different).
For all I know, I think that igoring the orthography, the distance between Manx and the other two Gaelic languages shouldn't be much larger than the distance between Irish and Scottish.
Manx is a Goidelic language, like Irish or Scottish Gaelic, unlike Welsh, Breton, and Cornish. It sounds a lot like Irish. In terms of revival, it's arguably in better shape than Cornish in that the Manx preservation movement started while the last native speaker was still alive, leading to a reasonable effort to record the details of the language while it was still a living language, whereas the last native Cornish speaker died largely unheralded, and it's known chiefly from an unfortunately small written corpus.
Of the extant Celtic languages; Manx, Irish, and Scotch Gaelic ar Gaelic, or Goidelic; while Welsh, Cornish, and Breton ar Brittonic, or Brythonic. Sadly, many other Celtic languages died out, including Celtiberian, Gaulish, Galatian, Pictish, and Cumbric (also Brythonic). (Wer Pictish and Cumbric essentially the same language?)
As for Cornish, when the Cornish revival movement started in the late 1800s / early 1900s, there wer still people with a residual knowledge of it, so we could reliably reconstruct it, and from these residual semi-speakers, we know how it was traditionally pronounced, so the pronunciation of revived Cornish is mor or less authentic, aside from the effects of sound change, which is a natural fenomenon and therefor does not harm authenticity. So don't worry; so far as knowing what the original Cornish was like so that we can revive it authentically, we ar good to go. We really ought to hav a Duo course on Cornish. Kernewek bys vyken!
That's correct. Both languages wer thought to be extinct (Cornish for perhaps as much as ~200 years), but both ar coming back. There is, for Manx, Bunscoill Ghaelgagh, and i hope they start mor Manx-medium schools soon. I started learning Cornish on June 1, 2019; even tho i don't believ i hav ever been to Cornwall and don't hav any close ties to it. Duo needs to become way mor aggressiv about creating new courses; there ar many courses that need to be created, incl. courses in Manx and Cornish.