Problem:
When adding more than one item to the cart, when not using the default language, warning message pops up asking if user wants to keep the selected language - if clicking Keep language the cart is deleted
Solution:
it seems that the W3 cache plugin was causing the issue. Though it was set not to cache the cart pages it seems to have happened anyway. By deleting the W3 cache plugin and replacing it with WP Rocket, the problem has been resolved