I’m sorry, but in the current plugin version, you should replace it with CSS. In the next plugin update, we will allow translating it by editing the .po files or with third-party plugins like Loco Translate.
Please enter the style definition below through the menu option “Appearance > Customize > Additional CSS”:
I’ll go in the meantime with this css hack, there is just a little correction to do (for eventually future users): it covers both the source and 0 results label, so 1 option would be: