Why not split the disscussion on the hardest language into four smeagols?
What languages should be nominated?
I think maybe (beta version of my thoughs) that let's say
Navajo has hard pronuntiation, also Abkhaz, as Abkhaz is Kartvelian language, Causasian language are the first languages of humanity.
In Basque the word for ceiling is "cave's top", also it is said that humans originated in Causasus (not as they long believed n Africa).
Also Germanic langauges are very hard for Slavonics to pronunce.
Grammar- I don't know, maybe Polish, or Yugoslavian.
Also Mongolian is quite tricky.
As for writting, maybe Yi, or some ideographic script as Nsibidi or Egyptian.
Chinese characters are bigger in number to Yi, but many of them are compounsds so it's easier to leared.
I don't knew.
maybe Chinese, so many words.