Just a passing remark:  A number of different kinds of concosmic
conlocations have been mentioned in this thread (distant planets,
fantasy settings, alternate universes, alternate histories with
various degrees of divergence, etc.), but there's one type which
hasn't been mentioned yet - namely, conlangs designed for actual
cultures which actually exist(ed) in our world.

An example of this is Brad Coon's Adena project.  The Adenan
Culture is a civilisation which existed in North America many
centuries ago (the Adenans were the original Mound Builders).
However, the Adenans disappeared before their language could be
recorded, and no-one is really sure even what family it belonged
to, thus leaving Brad free to invent a language (family) for them.

Has anyone else ever tried anything like this?

(On reflection, I suppose that reconstructions like Proto-Indo-
European might count as conlangs of this type, but of course what
I mean is completely fictional conlangs, attributed/assigned to
actual peoples whose real language has been lost.)