User blog comment:Kaido King of the Beasts/Chapter 818: Stepping Stones to One Piece/@comment-4374114-20160303195953/@comment-183.83.50.103-20160304071138

Well one theory (the one that I prefer) is that Dragon made sure that Robin's ability to read the language be hidden, at least to the big shots. He does have a lot of influence, and he obviously knew about Robin for a long time.