Manual:$wgDummyLanguageCodes/ja

詳細
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」はメッセージの説明文を記述するためのダミー「言語」です.
 * 「qqx」( 以降) はダミー「言語」であり、「 」を使用してインターフェイスのメッセージ名を表示する際に使用できます.

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).