In the context of the folktale in question, which is a variant of the folktale, ATU 328, ‘The Boy Steals the Giant’s Treasure’ (‘Jack and the Beanstalk’), I don't think we can regard the ability to smell a type of blood as referring to a specific ethnicity. The giant refers to an Englishman, because that is the only type of human who was likely to be coming to his place, which had its roots reaching down to England. The giant is saying, "I smell the blood of a man" - in other words - I smell a man and it is an odor I haven't smelled before within my own home. And keep in mind, we're talking about a giant with supernatural abilities, so while no one is suggesting in the folktale that people could discern a Frenchman from an Englishman, etc by the smell of their blood, it is possible for a giant to smell YOU - so be careful if you go climbing extraordinarily tall beanstalks.