Here is an interesting piece on the Spanish creds for a few political figures. Of course it's a Republican operative that brought up the fact that Julian Castro is not very conversant in Spanish (TBD?).
Will Speaking Spanish Matter in 2016?
http://blogs.wsj.com/washwire/2014/0...2016-hopefuls/
But reading about him, Julian Castro is quite an American success story and that is something the right wing will have difficulty knocking (rather like BHO).