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Hola Hombre!
What coopersita said is absolutely right, and needs to be checked on your site thoroughly.
Each module needs to have in its language folder a folder named spanish, with the appropriate php language files (most commonly main.php, admin.php, modinfo.php..).
BUT this might still leave you with a few words in englich (or what ever lang was the prefered of the module developer), as many modules do not adhear strickly to the creating language constants for all words/sentences displayed, but rather hardcode them.
To find them, you need to look at your site, then find the page where you don't have spanish, check the URL, and then open that file in an editor, and change them by hand.
Sorry for the inconvienence....hope this is the hint you were looking for....