A lot about difficulty depends on with what language you start off.
There's a difference between first language acquistion
Book of Words trilogy by JV Jones
Sword of Shadows series by JV Jones - the last book has yet to be published though supposedly it'll be next
Sword of Shadows book 5 by JV Jones (supposedly going to be released next year)
Stileto by Daniel O'Malley (sequel to The Rook)
I had no idea Interstellar was a movie that would be coming out - and was busily writing my own story about the subject, but from a slightly different
The English language I would have to admit is probably the most difficult language to grasp on paper, as there are a horde of rules and a lot of them