This is a big translation task, 47kb en-US.properties file, not for the faint of the heart. It took me a lot of time to make such a huge and hugely useful addon, along with numerous options and in-app documentation, hence the remarkable size of the file.
I wish someone with a lot of enthusiasm for the addon (and maybe perseverance too) would help translate the file to Spanish. I put the latest localized version of addon as well as the en-US.properties file at my website (I also attached the addon and en-US.properties files to this message).
If you finish the translation, please send it to me through email (found in the page above) or reply here and attach the file. Don't forget to let me know how you want your name displayed/credited in Fastest Search's 'About' link in its Options panel.
Thanks again for your translation effort!