One of the best methods for finding Spanish-Language websites through a Google search is to use Spanish words and phrases as your search terms. If you need English-language translations for the websites, on the results page, click on "Would you like to translate this page?" dialog box at the top of the page.
This will show you the English-language translation, but you can always click on "Show original to see the results in Spanish. See image below.
Another option for finding Spanish-Language websites is to limit your search to a domain from a Spanish-speaking country. For instance, the domain for Mexico is .mx. If you type site:.mx into your Google search box and then type in your search terms, you will only retrieve websites from Mexico. See image below. To find additional country domains, click here.