Once the wiki has been created, obviously.
Make sure that the language code appears in the file wmf-config/InterwikiSortOrders.php in the operations/mediawiki-config repo.(Example: https://gerrit.wikimedia.org/r/359810) (apparently no longer needed ad compact language links is deployed everywhere?
- Run populateSitesTable on at least all Wikidata clients (including wikidatawiki itself and testwikidata). (Done in T253756#6170109)
foreachwikiindblist wikidataclient extensions/Wikibase/lib/maintenance/populateSitesTable.php --force-protocol https
Beware: The script sometimes fails with a duplicate key conflict. In that case, go to the wiki's master database and empty the sites and site_identifiers tables, then run the script again. It's probably also wise to backup these tables from Wikidata and at least one Wikipedia before running the script across the whole fleet. Breaking the sites, site_identifiers tables will break page rendering of many wikis!