Daniel--How is the natural language of the horse Portuguese? If horses had any aptitude to actually learn language, it would be regional. It would be no different than any other animal, or human. No different than how German Shepherds are taught German commands in Academies, but can learn and respond just as well to English commands depending on where they are taught.
I also don't think it's unreasonable to believe that the sound of a familiar voice can soothe a horse. If it is of no consequence, why do they 'listen' at all?
I feel this is all pretty off topic anyway, since the question retained to a trainer speaking to a student, not a horse.
The sensitivity of the internet baffles me.