The Language Switcher module lists the content languages published on the site and lets a visitor move between them. It works from the language tags set on articles, modules and menu items.
When a visitor switches language, Joomla sends them to the Home page — or to the associated menu item — defined for the language they chose. From that point on they see the navigation built for that language.
The System – Language Filter plugin must be enabled for the switcher to work. Content languages themselves are managed in the administrator under System → Content Languages.