Localizations were often made by local publishers, in case of Spain that was Erbe Software. The English and German version were published by Renegade Software. Both companies ceased to exist when they were bought up in the 90s, but there's a good chance, that the rights to the different language versions still belong to different companies. When Flight of the Amazon Queen became freeware in 2004 it was explicitly the German and English version, so it were the former rights from Renegade, that covered the freeware status. This implies, that the Spanish localization probably still isn't freeware and therefore not included in the GOG version.