This is May from Shopify.
Currently, editing the 'Buy It Now' button or the dynamic checkout button is not possible as the code for this feature is core-based, which means that it is inaccessible. In case other merchants see this thread, and are wondering about how to translate this button to other languages, this button will be translated when you select a supported language through your theme. However, we do not yet support this through an individual field in the language settings.
That said I would be happy to launch a feature request with our developers to see if this functionality can be added in the future. While I can't guarantee that it will be added, I'm more than happy to pass along your suggestions that would provide better services to the Shopify platform. If the request is already there, I can add your vote to the existing ballot our developers' review so that they know there is further interest in getting it added.
If you have any further questions about dynamic checkout button, please let me know!