How to setup session sharing for language subdomains (URL language detection set to domain prefix) I would like to use domain prefix for language selection, like en.example.com and de.example.com. I've set up language detection to Domain here admin/config/regional/language/configure/url.

In theory everything works fine, except that session data is not shared between subdomains. If I log into the main site example.com and then visit en.example.com I have to log in again. Is this behaviour by design? Can I setup drupal to share session data between subdomains? It's the same site after all - only subdomain is different.
You need to set the variable $cookie_domain in your settings.php file to your base (default language) domain.

Just search the settings.php file for that variable, uncomment the line and replace '.example.com' with your default language domain preceded with a dot.

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