I dont know if this additional info can help but I checked the google chrome console > Network > XHR when loading the checkout page.
When Polylang is activated (checkout page not working)
Response is “-1”
When Polylang is not activated (checkout page working)
Response is :
{result: “success”, messages: “”, reload: “false”, fragments: {,…}}
fragments: {,…}
messages: “”
reload: “false”
result: “success”
This is all somehow linked to an Ajax query but I can’t say more than that :s…