**Feature summary** (what you would like to be able to do and where):
Support the apc language code.
**Use case(s)** (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):
I want to be able to use `language=apc` (for instance) in citation templates on the English Wikipedia.
However, there is a limited number of supported languages for citation on Wikipedia: https://en.wikipedia.org/wiki/Template:Citation_Style_documentation/language/doc
In particular, Levantine Arabic is not recognized whereas other Arabic varieties such as arz (Egyptian), arq (Algerian), aeb (Tunisian), shu (Chadian), and ary (Moroccan) are: https://en.wikipedia.org/wiki/Help_talk:Citation_Style_1#Add_languages
**Benefits** (why should this be implemented?):
Support one of the most spoken and most widely understood Arabic varieties in the Arab world.
According to {T187344}; "If one or the other language code is needed (for labels/descriptions/aliases) or just monolingual string or lexemes, it might be worth creating a separate request.", here it is!
See also: {T286290} & {T286417}
Adding languages: https://www.mediawiki.org/wiki/Manual:Adding_and_removing_languages
[x] Add to translatewiki
[ ] Add to cldr extension if possible or the cldr overrides
[x] Add to language-data if not possible to add to cldr extension
[x] Add to `includes/languages/data/Names.php` ([[ https://gerrit.wikimedia.org/r/c/mediawiki/core/+/955921 | patch ]])
[x] Set fallback, etc. ([[ https://translatewiki.net/wiki/User_talk:FunLater#Fallback_and_namespaces | discussion ]], [[ https://github.com/wikimedia/jquery.i18n/pull/278 | jquery.i18n patch ]], [[ https://gerrit.wikimedia.org/r/c/mediawiki/core/+/955921 | core patch ]])
[ ] Add to rtl languages of MF ([[ https://gerrit.wikimedia.org/r/c/mediawiki/extensions/MobileFrontend/+/1025703 | patch ]])
[x] Add to release notes ([[ https://gerrit.wikimedia.org/r/c/mediawiki/core/+/955921 | patch ]])
[ ] Remove from translatewiki after deploying the core change ([[ https://gerrit.wikimedia.org/r/c/translatewiki/+/1025708 | patch ]])
[ ] Remove the language from Wikibase and WikibaseLexeme ([[ https://translatewiki.net/wiki/User_talk:FunLater#Fallback_and_namespaces | patch ]])
[ ] Fix ULS language search
(Comment: The initial version of this task included the ajp code, but that code was deprecated in ISO 639 since then.)