Hi.
2 things - first: you are not searching for a Spanish source code, aren't you? Setting the front end to "espanol" is very simple. Log in -> settings -> first tab -> language.
The second: Are you sure, you will annoy people with ad banners?! Please, the fact that you can do this, means not, that you have to do it... everytime I am using a freemail service including an ad inside my mail, the first thing I do is removing it and please do not force your customers to have your ad inside after composing, so they can't influence and see it...