From personal experience
I would say Spanish.
(from "A Tramp abroad")My philological studies have satisfied me that a gifted person ought to learn English (barring spelling and pronouncing) in thirty hours, French in thirty days, and German in thirty years. It seems manifest, then, that the latter tongue ought to be trimmed down and repaired. If it is to remain as it is, it ought to be gently and reverently set aside among the dead languages, for only the dead have time to learn it.
Llaybach and Prestatyn !I went to a school in Cardiff in the early to mid 50's where learning the Welsh language was a compulsory subject and one that I really struggled with.
Surprised no one has mentioned Finnish which is also notoriously hard to learn.
If all these foreign Johnnies learnt fucking English we wouldn't have to bother and we'd have more spare time for drinking.
German is easy for people who already speak English.
I'd say backwards Russian is the hardest.
Nur ein kleiner Witz. Please open it againMein mund ist fest geschlossen