disount

Because you need to change the option "show discounted price" under the Display tab of the configuration (for the product page) and the same option of the hikashop options of your menus and modules (for products listings, via the menu Display->Content menus/modules).

You should be able to do that with a translation override (the strings related to the price display are at the top of the file):
www.hikashop.com/en/download/languages.html#modify

If you want it at the beginning, you can override PRICE_BEGINNING instead and it won't be stroked.

Then, if you only want it for discounted prices, you should use the key PRICE_DISCOUNT_START like that:
PRICE_DISCOUNT_START="<span style='text-decoration:none !important;'>On sale </span>"

Can you give a link so that we can see that ?

I see the problem. Even though the CSS is correctly applied, the browser still displays the stroke from the parent html element.
Then, you'll have to do differently.
Instead of that override, edit the file "listing_price" of the view "product" via the menu Display->Views and add the line:
echo 'On sale ';

after the line:
if(!empty($this->row->discount)){