/language/en/email. If you update the extensions your changes would be overwritten.
language/en/emailfolder to remove the markup. These changes will be overwritten when you update the extension.
/language/enfolder in the extension to
/language/esthen carefully edit the files to make the translations. It's not too hard to introduce a syntax error when making changes. You may need to purge the cache to see the changes. The Spanish language pack also needs to be enabled and your profile needs to use the Spanish language pack.
Could you add an option in the admin to remove these? I think it would confuse users more than anything...
Looking forward to that update to avoid confusing users as most don't even know what html is and probably email spam filters also would be happier without some of these words.