Just tried it, and it works fine. Although there was a typo in the BIKI : the Sides array is supposed to look like this:
RUG_DSAI_SIDES =["RUG_DSAIArab", "RUG_DSAIENG", "RUG_DSAISpa", "RUG_DSAISpa", "RUG_DSAIRus"];
My mistake, now fixed in BIKI too. Anyway, it does work on my end. Remember that the Spanish voices (like most other languages than English/Russian) mostly only has audio when in Danger mode - I think they have a select few Aware ones, but not many. So always try to see if it works while in Danger (Combat) mode first.
Btw: I put the above array in the init field of a unit when testing - if you still can't make it work with the init.sqf, try that. Or try to put it behind a sleep 0.1 or something.