I found it. Just as I thought the answer was very simple. There is a sub-directory called languages and a file for every language, such as english.lang.php in which there dozens of entries such as the following:
DEFINE('NOALERTSDETECT','no alerts detected');
DEFINE('USEALERTDB','Use Alert Database');
DEFINE('USEARCHIDB','Use Archive Database');
DEFINE('TITLE','Basic Analysis and Security Engine (BASE) '.$BASE_installID);
DEFINE('_FRMLOGIN','Login:');