I was asked in a "private message" to explain my understanding of mandante and mandatario. It seems to me that putting it in the legal forum is the best place to elicit explanations from other participants native and more knowledgeable than I. A mandante is ultimately the subject who manda, and the mandatario is the indirect object of the verb mandar. Primer mandatario has become a cliché for "head-of-state" or "highest authority" in Latin American newspapers, which stand on their heads to avoid repetition, inventing bizarre and overblown synonyms such as líquido precioso for "water" in an article about water rates or broken water mains. Saying "Presidente" more than once seems to offend the ear, so the dictator of the moment becomes "el primer mandatario de la República," etc. A mandatario is one "sent" or "ordered" or "empowered" to govern, as for example a member of the Legislature. But the term primer mandatario in the authoritarian Latin tradition has come to be understood, for the presidente of the moment, the first among those who "give orders," not the chief among those authorized by the mandate of the people to govern. In my experience, the grantor of poderes is as frequently referred to as el poderdante as el mandante. A mandatario can be an "attorney" at law, a lawyer who represents a client in legal transactions, or an "attorney" in fact, any individual authorized to act on behalf of another. As for their specific translations of the terms into English, absent any context, I can´t say without seeing their uses. However, attorney-in-fact, the example you cite, is a very common phrase in English legal documents, and would be an appropriate translation for mandatario in certain contexts.