it seems that "delete selected" string can't be translated. it is used in various places, including trigger dependency deletion etc.
a quick grep reveals :

./frontends/php/include/forms.inc.php: array_push($delay_flex_el, new CButton('del_delay_flex','delete selected'));
./frontends/php/include/forms.inc.php: array_push($delay_flex_el, new CButton('del_delay_flex','delete selected'));
./frontends/php/include/forms.inc.php: $dep_el[] = new CButton('del_dependence','delete selected');
./frontends/php/include/forms.inc.php: array_push($dep_el, new CButton('del_dependence','delete selected'));
./frontends/php/config.php: array_push($valuemap_el, new CButton('del_map','delete selected'));