Can we translate the app into another language?
Yes, you can translate all customer facing copying into another language such as Spanish. However, the app does not currently support multi-language. You can also translate the app into any active language on your store - if a customer checks out in that language, your overrides will be used.
To translate custom facing copy, go to Settings > Translate:
How do we translate the post purchase header text?
Shopify inserts a mandatory header on the post purchase page and the copy can be edited within your admin's theme language:
Go to Online Store -> Themes -> Live theme -> Actions -> Edit languages:
Then search for: "You've paid for your order." or "View order confirmation":