There is very little built-in text in the forum. By "built-in text" we mean text that is hard-coded into pages and can only be changed by making changes to the source files. In fact, almost all the text that you see (apart from forum text and titles and such) is stored as phrases, and there is a great deal of logic behind separating the code of the forum from the text.
To begin with, it makes it easy to create language packs for the forum that customize the base language of the forum. There are in fact a number of language packs that have been created for vBulletin, including French, Spanish, Arabic, and Hebrew. Links to a number of these can be found on the official vBulletin site at http://www.vbulletin.com/forum/showthread.php?t=99151.
Another major advantage is that, in order to make a change to the text in the forum, an administrator does not need to change critical files containing PHP code, where a single error could bring down the forum. It is far safer and easier...