from the NetBSD maintainer:

This voice provides a Castilian Spanish male voice using a residual
excited LPC diphone synthesis method.  The lexicon is provived by
a set of letter to sound rules producing pronunciation accents and
syllabification.  The durations, intonation and prosodic phrasing
are minimal but are acceptable for simple examples.

This voice can be activated via (voice_el_diphone) .

WWW:  http://www.cstr.ed.ac.uk/projects/festival.html