To translate the default Shopify invoice design, you would need to edit the code for the template itself to change the wording. You can do this on Step 1 of the setup wizard.

If you are not comfortable with code, then I can recommend my other app Order Printer Templates: https://apps.shopify.com/order-printer-templates

All of those designs are compatible with the Shopify Order Printer app (for printing and saving documents yourself) as well as with our Order Printer Pro app.

It will allow you to translate and customize the documents, without needing to mess with any code directly.