Manual:$wgDummyLanguageCodes/de

Details
This global variable contains an array of language codes that don't have a corresponding set of translated system messages and never will.

These codes are left over from renames, or other legacy things.
 * In most cases they simply map directly to another language code.
 * "qqq" is a dummy "language" for documenting messages.
 * "qqx" (since ) is a dummy "language" that can be used with " " 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  maintenance script (used for generating localisation statistics).