To switch language (english to japanese or japanese to english). That example work with the plugin polylang activated and the japanese page and english page created.
Front-end( the page visible for the customer):
The english page will be shown if english
If the customer choose japanese on the menu so the japanese translation will appear
Back-end( Administration page):
To shown the administration page in japanese, you have to turn wordpress language in the setting in japanese, then the administration will be in japanese.