I can’t add the language switcher in the menus

In Appearance >Menus, click on screen options, top right of your screen, to expend the options and check the “Language switcher” checkbox. You should then see a new metabox in the “Add menu items” list 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.