Chouby on "[Plugin: Polylang] Setting of homepage"

I see the problem. But that's not something I reproduce. Could you try the debug mode to see if some error is logged?

In wp-config.php, replace:

define('WP_DEBUG', false);

by

define('WP_DEBUG', true);
define('SCRIPT_DEBUG', true);
define('WP_DEBUG_LOG', true);
define('WP_DEBUG_DISPLAY', false);

Then reproduce your problem.
Then go look in wp-content directory to see if there is a file called debug.log with error messages in it. Please report these errors.