Hi there,
Since you have a business edition of Hikashop, you can go to System>Emails to see a listing of the mail you just named.
Click on the name of one of these mail and you'll be able to see stuff like:
echo JText::sprintf('ORDER_CREATION_SUCCESS_ON_WEBSITE_AT_DATE',$url,HIKASHOP_LIVE, hikashop_getDate(time(),'%d %B %Y'), hikashop_getDate(time(),'%H:%M'));?>
The text in capital letters is the text which will be translate. You can found it in the language file as my colleague says above.
Just change this translation to change the content of the mail.
Be careful to keep the %s, this character will be replaced by the argument given in the sprintf() function