I would have the texts on a DB and find out which language the browser seeing the page has with a simple php function. Depending on that I'll show the texts in spanish or english. I would neither build a whole new version of the site nor create a subdomain.
If you do it like I say you will have the possibility to have your site in many languages just adding the texts to the DB, so it'll be able to contemplate more languages in the future.
