Why can't I segment and/or fully translate the order notification emails?

Order notifications are not fully translatable. Indeed, as indicated in the Mailchimp official documentation:

"The body of the Order Confirmation, Order Invoice, and Shipping Confirmation includes order information that cannot be changed."

"This is to make sure your notifications are compliant with laws regarding transactional and marketing emails, as well as Mailchimp's Terms of Use."

However you can add your own message, change the logo and the title (but you can only use a certain number of characters). To know more, please read this documentation.

Moreover, Mailchimp has not yet made it possible to send different order notification templates to different list segments. Therefore, you cannot plan to send a certain e-mail template to one segment and another to another segment of your list. The only possible "segmentation" is to add or delete, after activation of the automation, some contacts / segments of your list in order to limit the sending to some contacts (but all these contacts will receive the same e-mail template). For more information, please read this documentation.