Hi Kate,
This type of messages can be translated but they're not fully localizable (can't display dynamically the message based on the current culture).
The scenario is the following, if you have a server/farm that serves employees that speak Spanish then you can translate the message and they'll see the message in Spanish.
To do that go on each WFE server in your farm, open the file WAListForm.js from the following location (this can be different based on your configuration)
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\wpresources\WA.ListForm\4.0.0.0__2b4b09f1c57c8f0b, localize the message and replace it with your new message.
The above can be applied on all js messages in the wpresources WA* and Quest* folders.
Please note that the users have to clear the browser's cache to see the change.
Before you do any change please backup these files.
I hope this helps. If you still have any questions please let me know.