SGAE - Sociedad General de Autores y Editores : I doubt whether the latter is the Mandante in this case, though we don't have enough information on the underlying transaction: grantor ofa copyright licenc/se or assignor as the copyright owner or 'originator' to whom the copyright can be re-assigned e.g. a company employee. Otherwise, the following online quote would appear muddled:

'What is copyright? the exclusive legal right, given to an originator or an assignee to print, publish, perform, film, or record literary, artistic, or....'

if mandato expreso is being translated e.g. as an explicit agency, then the parties thereto would - in BrE / IrE - be principal and agent and the matter thereby resolved.