There are two ways….using a program like PoEdit to create a new translation (even if it’s still in english), and open the theme’s seasonalpro.pot file to make your translated version of that one language string. The other way is to switch to using a child theme for Seasonal Pro and edit the searchform.php file which has that “Search” language string in it.
Quick follow up to my last message…if you switch to using a child theme, please be aware that any option settings you did like colours, etc., will have to be redone from the customizer because activating a child theme will be seen as a new theme. But, the benefit here is that you can easily change theme files.
You could edit the header.php file , but it would be best to switch to using a child theme then. This would allow you to keep getting theme updates without losing anything you change, such as the search.
If you decide to switch to using a child theme, you would copy the theme’s header.php file and put that into your child theme, then you can edit that file there. The “Search” you see is the placeholder and it is found near the bottom of the header.php file.