I have tried my luck for many hours now, but unfortunately without success. I have found the delivery date string in the "accouting" module. But the correct translation is also given here.
Unfortunately, I don't know why it is not used.

First founded Translation

Second foundet Translation:

I found also the Translation für den PDF generation. But it dont work.

Any other ideas?

Hi,
Activate the German language,

1) Go to Settings ‣ Translations ‣ Import / Export ‣ Export TranslationsLanguage: Germanselect the PO File format
select your moduleclick Export and download the file.
2) Add your translation terms to the downloaded file. i.e. msgstr "" section depending on your translation language
4) create an i18n folder under your module and add po file in it.

For editing the delivery date, follow the code
#. module: sale_management
#: model:ir.model.fields,field_description:sale_management.field_delivery_date
msgid "Delivery Date"
msgstr "Liefertermin"
Replace 'sale_management' with your module name and '_delivery_date' with the appropriate field name for translation in the code.

For more details, refer to the blog:
https://www.cybrosys.com/blog/configure-translation-to-custom-module-add-new-language-in-odoo-16

Hope it helps