The "Confirm Order" after submitting 'Billing form' page is using the global
language setting, not the lang='' parameter in {Orders} tag.
Even though the lines are using " $orders->Lang ".

Secondly, the "Edit" link
Code:
{$back_link_url}
in the confirmation form returns to the page which is displayed also in "Default
language for the frontend"

So problem occurs if the site has 2 or more languages.