First export from translatewiki with https://gerrit.wikimedia.org/r/c/mediawiki/core/+/1270285
- Verify autonym correctness in language-data
- Add to Wikifunctions's language list in function-schemata: Z1194
- Add to includes/Languages/Data/Names.php
- Add to RELEASE-NOTES
- Add MessagesFrs.php
- Add fallback language if needed
- Also add this in Leximorph (includes/libs/Leximorph/data/language-fallback-mappings.json)
- Also add it to jquery.i18n (src/jquery.i18n.fallbacks.js)
- Add namespace translations (requested here)
- If a fallback language has gendered namespace aliases and this language doesn't, unset them
- Add $linkTrail if needed
- Add date formats (if needed)
- Add digit conversion (if needed)
- Add fallback language if needed
- Add an entry to LocalNames/LocalNamesEn.php in the CLDR extension (if needed)
- Define wider line-height in resources/src/mediawiki.skinning/i18n-headings.less (if needed)
- Remove from translatewiki after deploying the core change
- mw-config/LanguageSettings.php
- mw-config/AssistantLanguageSettings.php
- Test language search in UniversalLanguageSelector and fix the search index (if needed)
See documentation on how to add a language to MediaWiki core https://www.mediawiki.org/wiki/Manual:Adding_and_removing_languages#Core_MediaWiki