How do I translate/change the text of the order receipt email in Commerce 2?
You can find the Subject on the UI translate page (/admin/config/regional/translate).  Search for Order #.

The rest can be found in a twig template:

/modules/contrib/commerce/modules/order/templates/commerce-order-receipt.html.twig

Copy this template to your theme and then edit as desired.  You can also use the template as a guide for what to search for on the translation UI screen to translate everything.