It is possible to add new languages in LibreOffice 5.3 and later. There is a command line program called oxttools that creates an .oxt for a language. When the .oxt is added to LibreOffice, XML files are added that describe the language for spell checking and other language functions.
It expects a language tag, and there is no existing JSV tag. However there is "jsl" (normally for Japanese Sign Language). So what you can do is use "jsl" as the tag and then "JSV" as the language name, which is what will be visible in LibreOffice.

If you want to try this, here is the powershell command I used for this test.
& ./makeoxt -d wordlist.txt -l "JSV" -t west jsl out.oxt
Disclaimer: I am associated with the people that develop oxttools and have contributed code to the project. However it is free, so I am not profiting by advertising it or anything like that.
makeoxt.exeon my Windows 7. It seems the program requires 64-bit Windows, right? – user90726 Dec 29 '21 at 22:56