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":