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.

For multilingual

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