Why do you refer to a miraheze.org page as "The Interslavic Wikipedia community"?

Whichever discussion is necessary, we can (re-)do on Wp/isv. Are there any other issues to address before proceeding?

Change #1249402 had a related patch set uploaded (by Srishakatux; author: Srishakatux):

[mediawiki/core@master] Add language fallbacks and MessagesIsv.php for Interslavic

https://gerrit.wikimedia.org/r/1249402

Change #1249402 merged by jenkins-bot:

[mediawiki/core@master] Add language fallbacks and MessagesIsv.php for Interslavic

https://gerrit.wikimedia.org/r/1249402

When updating the search index

php maintenance/run.php CirrusSearch:UpdateSearchIndexConfig

we see the following message

<Error, collected 1 message(s) on the way, no value set>
+----------+---------------------------+--------------------------------------+
| error | rawmessage | This script encountered an index dif |
| | | ference that requires that the index |
| | | be
copied, indexed to, and... |
+----------+---------------------------+--------------------------------------+

and debugging that revealed that

isv_plain: not found...cannot correct

can this be related to https://gerrit.wikimedia.org/r/q/Ib0ebd292ab81685e296e4570d7205ab17cda5893 or are we just abusing the UpdateSearchIndexConfig?
I wonder if something similar had to be done within WMF?