The single sign on (SSO) feature, available in Polylang Pro only, allows you to be automatically logged in across multiple domains or subdomains. Here are some situations where this feature is useful:
You are able to preview translated posts in context. Without SSO, previews are displayed on the main site.
You can see visit pages in a deactivated language (when logged in with sufficient privileges). Without SSO, deactivated languages are not visible.
The admin bar is displayed on all domains (or subdomains). Without SSO, the admin bar is displayed only on the main site.
How it works?
Let’s say that you are logged in on en.mysite.com and you want to visit fr.mysite.com. You click on the link to the French translation in the language switcher and:
The French page is first displayed without you being logged in. The admin top bar is not displayed.
Polylang automatically redirects you on the same French page with you being logged in. The admin top bar is displayed.
Of course, the redirection does not occur for the next pages you will visit in the same domain or subdomain.
Notes:
When using multiple domains, this feature requires SSL.
Logout is automatically done on all domains or subdomains at the same time.
Polylang is not able to automatically sign you inside the customizer.