Beyond any user-created languages, there are 2 that always exist. These are master languages and are generally hidden from being directly edited.
The Master Language is where all of vBulletin’s untranslated, default phrases exist. This allows an original copy of a phrase to exist at all times for reference purposes. You will never be able to edit this language directly.
The Custom Master Language is similar to the Master Language. However, the only phrases that will be inserted here are any custom phrases that you have created. This allows every language to have access to the phrase as if it were in the Master Language.