Don here from Shopify!
I'm not totally sure I understand your request here though I might be able to provide some useful info. :)
You can check for any editable language in your theme by heading to Online store > Themes > Actions > Edit languages.
Have a look at the 'checkout & system' area here to see what language exists in the checkout area that you can edit yourself.
Other than this, it would not be possible to add something to or edit the checkout area for security reasons.
There might be some existing language there that you can edit to include the message you need, though it would not be possible to introduce an entirely new message here.
You can always reach out to us via our support site if there's anything else you need. :)
All the best!