All of the Facebook widgets (Like button, Fanbox, Recommendation, Invitation module, etc) are all automatically translated to the language you have selected in Joomla. This is done by Facebook automatically, and covers most language strings.
For things like the Login module and Registration page, those do require separate language files for Joomla. Right now, we don't include a Spanish language translation for JFBConnect in the package, but we do support the Joomla language file system completely. For JFBConnect, we have only about 20 strings that you'd need to create to support Spanish, and you're welcome to contribute those language strings back so they are included in the next release. Currently, we support French, Chinese, German, and Dutch, which have all been contributed by current users.
Hope this helps,
Alex