If you want to take a shot at doing your own English translation of a Spanish-language song, here are some sites that can help you do that.
Babelfish is the translation site that has been on the Internet the longest. You simply type your song lyrics into the dialog box, select Spanish-to-English translation, and submit. If the song lyrics are on the Web, you can also enter the URL into Babelfish, and the service will translate the entire page. When you've gotten a translation, Babelfish even provides an option for you to search the Web using the translated text. This is a good way to check your translation's accuracy: sometimes Babelfish can miss nuances of meaning or skip words it does not recognize, so you'll get a patchy or faulty translation. Searching with translated text lets you find other translations of your lyrics and compare them.
Google Translate is another automatic online translation service. It also gives you an option to search the Web using your translated text. And if you're not quite sure if the song you're translating is indeed Spanish, Google Translate has a "detect language" feature that will verify it for you.
It may be hard to do your own English translations of Spanish songs unless you have a good command of the language. Many popular songs will use slang or idioms with which you may not be familiar and that conventional online translation services may not be able to translate. Luckily, there are plenty of places on the Web where you can request that a native Spanish speaker do the translation for you, or where you can find a translation has already been done.
AllTheLyrics.com has a Spanish Lyrics Translation forum where users request English translations of Spanish songs. Users translate everything from reggaeton star Pitbull to Latin pop veteran Miguel Bose. There are often lively discussions about different forum users' interpretations of specific slang terms, or exactly what word an artist is singing. There's also a "stickied" (permanently promoted to the front page) post listing all the Spanish songs already translated on the forum, and providing links to the threads where they've been translated.
LyricsTranslate.com also has a Spanish/English translation forum, but traffic there seems to be light, with few replies to posts requesting translations.
Learning By Singing Spanish is a blog run by a woman who does her own Spanish-to-English translations of popular songs. She groups them by artist and also by country, so that you can see the differences in Spanish dialect from place to place. She accepts requests for song translations.