You can enable/disable an option to switch WordPress localization when the language is changed in the front-end. However, in the back-end, WordPress localization corresponds to the default language, and cannot be changed.
In order to enable the automatic language switching of your WordPress website elements as well as the page content in the front-end, mark the checkbox next to "WordPress Language" option on the plugin settings page. Installed WordPress language packs are required.
For having the translation of your theme and each menu item provided by WordPress, please go to "Setting" => "General" => “Site Language” option and make sure that the needed languages are present in the list of installed languages.