Currently ACT rules allow some fictional languages as valid languages because the IANA registry contains some fictional languages:

https://act-rules.github.io/rules/de46e4#valid-language-tag

BCP-47 fictional languages:

tlh - Klingon (Star Trek)
sjn - Sindarin (Lord of the Rings)
qya - Quenya (Lord of the Rings)

Only a few fictional languages have BCP-47 codes, others like Dothraki and Vulcan don't.

https://www.iana.org/assignments/language-subtag-registry/language-subtag-registry

A related issue are extinct languages with BCP-47 codes but which will never have AT support:

nrn - Norn - was spoken on Scottish islands until 1850
xpi - Pictish - was spoken in mainland Scotland until 1150

Lots listed here with BCP-47 tags:
https://en.wikipedia.org/wiki/Lists_of_extinct_languages