yep lines 387 to 390

Code:
// for all pages except where multiple add to cart is used:
  define('SUCCESS_ADDED_TO_CART_PRODUCT', 'Successfully added Product to the cart ...');
// only for where multiple add to cart is used:
  define('SUCCESS_ADDED_TO_CART_PRODUCTS', 'Successfully added selected Product(s) to the cart ...');
weird huh?