If you want to add the invoice due date in your email template, you will need to generate and use dynamic placeholders.

First, make sure the developer mode is active. Then, search for your email templates in Settings / Technical / Email / Templates, and open the template you wish to adapt (in my case Invoice: Send by Email)

Generate your due date placeholder and copy/paste it in your email body.

Though your date format settings are set up properly in your language settings, you see that the standard format for this placeholder is YYYY-MM-DD.

In order to use the DD-MM-YYYY format, you will need to use the following placeholder: ${object.invoice_date_due.strftime('%d/%m/%Y')}