This is currently only available in beta.

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.