Hi,
piBase only loads one file (hardcoded locallang.xml)
You can read any lang-file:
$myLang = $GLOBALS['TSFE']->readLLfile('EXT:myExt/locallang_db.xml');
You can output any single label by using lang method:
$GLOBALS['TSFE']->sL('EXT:myExt/locallang_db.xml:labelname');
vg Steffen