Hi!
Just a precision since polylang5.png shows the quick edit mode. If you change the language in quick edit, the edited page will loose all its translations. That's intentional. Is your problem coming from this?
Otherwise, can you try to switch to a default theme like twenty twelve, de-activate all your other plugins and restart the operation only with polylang active? If it works, then you can try switching back to your theme and activate your plugins one by one to find which one conflicts.
