Bottom line -
(while Davidglen may be right, it is unlikely that an interpreter at the ceremony itself will make people think twice about the issues he mentioned. Those who are up to thinking this through, will have done so by that point.)
- at the ceremony the judge explains your responsibilities towards each other, and a bunch if other stuff. Both the groom and bride are asked if they understand and accept this, and they respond "Si, acepto".
It's kind of expected that the chief participants understand what they're being told, and if it's understood that they don't, an interpreter must be brought. All in all, this is not illogical. They don't make money off this, the interpreter is paid his/her fee and that's it.
In my case, on one of my visits to the registro civil prior to the ceremony, I was administered an on-the-spot Spanish test which consisted of: "donde vivís?", -"donde naciste?", "en que fecha naciste?", and "trabajas acá?". Having passed that (I'd been in the country for a few months at that point), I kept my mouth shut at the ceremony except to say "Si, acepto".