How does PHP Nuke do it? vBulletin uses "phrases" which are strings stored in a database. The phrase can be made in any language and vBulletin loads those phrases into an array and calls that variable. IE:
PHP Code:
echo $phraseDB["somePhrase"];
Similarly, Joomla 1.x (not 1.5) does it the same way except with files and it does not use an array to hold the vlaues.