This thread is resolved. Here is a description of the problem and solution.
Problem: When i go to the Mongolian version of the site and add an item to cart and go to checkout, the checkout will show an error message saying "Sorry, your session has expired. return to shop"
Solution: - Go to WPML -> Languages -> Language filtering for AJAX operations section -> Activate the option and save the changes.
This is the technical support forum for WPML - the multilingual WordPress plugin.