I think you should make it in both languages. I've done that in the past (I also speak Spanish). All I did was make a splash page and give people the choice of English or Spanish.
Or you can just do it in english. If you don't think you'll get a lot of Spanish visitors, then taking the bilingual approach is wasting time on your part.
It depends on who you actually want the site to be for. If it is a general interest site, then I would go for both languages, or english if you had to pick one. But if it is...say, a spanish politics discussion site, The hell with all those gringos, write it entirely in mexican.
Disclaimer - I am more than aware that the boy is probably NOT a beaner. I was making funny. Enjoy.
I'd suggest to do it with English/Spanish so the visitor can chose. The more option you give the more visitor can see your stie.
I also vote for doing it in English and Spanish. The set cookie idea is great, the two flags at the top of every page is great. No one likes to click through a splash page to get to actual content, so if they only have to do that once, terrific. Actually, I would advocate for setting the default home page in one language or the other and make it clear that you can switch languages at anytime. That way, first time visitors wouldn't have to load and click through a barrier to get to the content.
Go to google.com/translate.
enter the page on your site you want to translate.
You will get the link on the address bar .
On your site, put up a link named spanish and link it to the google translate page which you copied earlier.
When people click on it they will get the spanish translation
english, french, spanish, italian, german and chinese