I am certain your developers are doing the best they can to solve the issue with Cornerstone and Polylang. For me too after the update of WP, X theme and Cornerstone I can't edit pages and posts within the language folders /en, /de, /sv. It is an awkward situation because it delays the development of my website.
The Dutch pages can be edited with Cornerstone. All pages within the language folders having the message below:
"The preview could not load due to the iframe response being incomplete. This is most often related to a plugin conflict, or customizations introducing a PHP error."
Do you have any further information about the issue being fixed?