It is possible to create AutoCorrect replacement files for any valid BCP 47 language tag resulting in, for example, some autocorr/acor_art-x-privuse.dat file in the user profile (see https://ask.libreoffice.org/t/how-to-create-an-exportable-keyboard-input-scheme/80482/4 )
Dropping such acor_*.dat file into the autocorr/ directory should add the language tag to the list of available languages so it could be readily used without the kludge of needing some source-edited template file just to be able to assign the language.
Eike Rathke committed a patch related to this issue.
It has been pushed to "master":
Resolves: tdf#150371 Scan autocorr dirs for language tags to add to list
It will be available in 7.5.0.
The patch should be included in the daily builds available at
https://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More
information about daily builds can be found at:
Affected users are encouraged to test the fix and report feedback.
I could verify that new languages can be listed if a config file was manually added, basing it on an existing language's .dat archive and renamed. Could add new entries to it with the GUI.
Version: 22.214.171.124.alpha0+ (X86_64) / LibreOffice Community
Build ID: 01c9c971e43782800ebf63acc763a7e7fba096c1
CPU threads: 8; OS: Linux 5.15; UI render: default; VCL: gtk3
Locale: en-AU (en_AU.UTF-8); UI: en-US