Manual:$wgExtraLanguageCodes

This array makes the codes not appear as a selectable language on the installer, and excludes them when running the script.

In, the variable is created by combining these codes with a list of "deprecated" codes, which are mostly leftovers from renames or other legacy things, and the internal codes 'qqq' and 'qqx'.

A mapping in  takes precedence over built-in mappings.