Hello, @manos,
You can change that phrase from XStore >> Theme Settings >> WooCommerce (Shop) >> Shop Elements >> Sale & Out of Stock & New:
From there you can change the Sale Label Text and you can turn ON/OFF the show percentage as well.
https://postimg.cc/2V9hbZP7
https://postimg.cc/7G428M2w
Regards 8Themes Team.