https://paste.ee/p/CX3pW

Going back to fabric-language-kotlin-1.3.30+build.2 solves the problem.

Happens with no other mods installed (besides FabricAPI):

https://pastebin.com/7Y3bpWy8

Same. Caused by jar-in-jar not working properly?
MultiMC log

I have archived the file on curse, @NikkyAI when you get a chance mind taking a look at this?

seems like moving to jar-in-jar did not work as intended, might it be that the language adapter itself requires stuff to be shadowed ?
weird that it seemed to work for me

yeah the problem seemed to have been that i overlooked one dependency, shadowJar added it transiently and jar-in-jar does not
this also explains the filesize difference, should be fixed in build.4
i tested in multimc with the following mods:

• [email protected]
• [email protected]+build.127
• [email protected]+build.154
• [email protected]+local
• [email protected]
  and apart from flooding modmenu with all the library entries.. it seems to work fine