I have a webform in spanish and I have manually put in all of the validation error messages in spanish, however I cannot find where I can make the "Please enter an email address" message which comes out if you put an invalid email into an email type field come out in spanish instead.
Try with String Overrides module.
When you go to module settings choose 'Spanish' tab (if you have more than one language enabled), on the left side put "Please enter an email address" (without quotes of course) and on the right side write Spanish translation. (clear cache)
Let me know if this is working.
another way I found to do it: Go to configuration, translate interface, do a search for the error message in english and then add the spanish translation.
I just changed it to a text field instead of an email field and validated it with regex so I could put the error message in spanish.