I think this question has prb.
What is the most difficult? We cant answer this Q .Why? Cos it depends on what language you are speaking and what foreign language you are studying.
If your mother tongue is like foreign language you are learning in some aspects , It is easily for you to learn.
For example:With Chinese and Vietnames : Their pronunciation is nearly the same .So it is very easy for Chinese to speaking Vietnamese, and Vietnames are easily speak Chinese. But about writing , it is very difficult .Cos Vietnamese writing includes Latin characters while Chinese writing is pictographic.Maybe with Indian , Japanese ppl, Chinese writing iis easier?
With Vietnamese and English , the prb is completely different!
it is reading!English hasn't tone , but Vietnamese has.So Vietnames is ponderous.