Borges use of the Spanish language was perfect. He had a very deep knowledge about the precise meaning of words native speakers use in a more careless way. And

Borges use of the Spanish language was perfect. He had a very deep knowledge about the precise meaning of words native speakers use in a more careless way. And he had the accent, etc. of a native speaker. People might find it hard precisely because he uses the language with an exactitude and a vocabulary that is really uncommon.