Add header image to each page:
Appearance -> customize -> Main Content -> Single pages
then change the setting post thumbnail position to before the title in full width.
This theme not support Polylang , So I am using weglot . It can translate one language for free.
To add language switch to the menu: You can place the button in a menu area. Go to Appearance -> Menus and drag and drop the Weglot Translate Custom link where you want.
For change menu color and body color
you need edit the /booking/wp-content/themes/customizr/assets/front/css/style.min.css
menu change flex-lg-column element
For other setting