Hi, I had the same issue. The word is in “jak2_filter.php” (plugins/search directory).
Change the following lines:
line 563 : $objects[]= JHTML::_(‘select.option’,””, JText::_(‘<fill in your new “all” text here>’));
line 624 : $mitems[] = JHTML::_(‘select.option’, $groups[0].’_0′, JText::_( ‘<your search all categories text here>’ ) );
line 688: $mitems[] = JHTML::_(‘select.option’, $groups[0].’_0′, JText::_( ”<your search all categories text here>’ ) );
line 697 : $mitems[] = JHTML::_(‘select.option’, ‘0_0′, JText::_( ”<your search all categories text here>’ ) );
On example line for line 624 with the dutch translation:
$mitems[] = JHTML::_(‘select.option’, $groups[0].’_0′, JText::_( ‘Alle categorien’ ) );
This did the trick for me.