The 'Master Language' and 'Custom Master Language' 
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.
Natch 20th Apr 2005, 11:06am
Note: if you have debug mode enabled, it the default behaviour, and thereby is *very easy* to be adding phrases to the MASTER LANGUAGE, thereby causing your carefully crafted phrases to be overwritten on upgrade of your vBulletin.

There is a small hack [vbulletin.org thread 67408] of your AdminCP which will make the default behaviour in debug mode to add phrases to the CUSTOM MASTER LANGUAGE, instead of MASTER LANGUAGE - this will mean your phrases will be retained in the database AND available to all Custom Languages when you upgrade.

HTH - thanks for listening.