In Appearance >Menus, go to screen options on top right of your screen and check the 「Language switcher」 checkbox. You then should have a new metabox which allows you to add a language switcher the same way you add other menu items.
Due to WordPress limitations, it is currently not possible to add the language switcher from the Customizer Menus.
I can』t add the language switcher in the menus
I can』t add the language switcher in the menus