template\scripts\BxBaseMenuFooter.php <<< line 54ish ...

case 'powered_by':

                   $sTitleAttr = bx_html_attribute(_t('_sys_txt_powered_by'));

Some other language keys:

_powered_by

_sys_page_lang_block_about

_sys_menu_item_title_system_powered_by

_bx_artificer_txt_splash_welcome_l4