A second language is a lot of work both to setup and subsequently to maintain. With the continuing improvements to translation tools like Babelfish you are better off pointing your users to such tools and then your website can be in many languages not just two.
Your site should be in the language primarily used by your target audience, or if you have any doubt as to what that is, then use English.
For
SEO purposes it may be sensible to have a couple of Spanish paragraphs on your home page with relevant keywords in. Aside from that I wouldn't waste time on it.