Manual:$wgDummyLanguageCodes/en

Details
This global variable contains an array of language codes that don't have a corresponding set of translated system messages and never will.
 * In most cases they simply map directly to another language code. These codes are left over from renames, or other legacy things.
 * "qqq" is a dummy "language" for documenting messages.
 * "qqx" (since ) is a dummy "language" that can be used with "&uselang=qqx" to view the message names in the interface.

These language codes are excluded on the installer as selectable user or content languages.

Since, they are excluded when running the transstat.php maintenance script (used for generating localisation statistics).