The 'Master Language' and 'Custom Master Language' 
Regardless of any languages you enable as administrator (for example, Spanish or German) there are always two that always exist in vBulletin: the Master Language and the Custom Master Language. They are generally protected from being directly edited.

The Master Language is where all of vBulletin’s untranslated, default phrases exist. It is important to have the original version of all phrases available for reference purposes. You cannot directly edit these language phrases.

The Custom Master Language contains only custom phrases you have created, which can be accessed as if they were part of the Master Language.
User Contributed Notes: The 'Master Language' and 'Custom Master Language'