Tira here to help.
Thanks for reaching out and I really appreciate you doing your own research on this. That’s correct, you won’t be able to edit the language for Apps through the Language Editor in your Shopify Admin. You can edit the language for different fields in your checkout by following the steps from our help doc here: Translating the checkout.
If you’re looking to change the text of the “Download Now” button for the Digital Downloads app that is made by Shopify, this isn’t a feature that’s available. I would be happy to forward this as a suggestion to our developers so that they are aware, and can work on improving Shopify for merchants like you.
To help us understand what you need and what we can work on, I would love to hear more about this. What were you hoping to change the text to?