What you're proposing is not an objective measure, it's a popularity contest.
And yes, in a lot of such surveys you'll find Chinese/Japanese/Korean sitting at the top of the list. Sometimes with Arabic.
Maybe Japanese is a really hard language objectively, but these surveys aren't actually showing that. What they are showing is that the majority of organizations that are doing these kind of surveys are populated by speakers of western European languages, who find Japanese "objectively" much harder than Spanish.
That list is the CIA/State Department evaluating the average time to fluency for English natives going through one of the foreign language training programs they run.
Chinese, Korean, Japanese and Arabic sit on top of the list because they are the most isolated away from English.
Koreans find Japanese extremely easy to learn. Vice versa.
And yes, in a lot of such surveys you'll find Chinese/Japanese/Korean sitting at the top of the list. Sometimes with Arabic.
Maybe Japanese is a really hard language objectively, but these surveys aren't actually showing that. What they are showing is that the majority of organizations that are doing these kind of surveys are populated by speakers of western European languages, who find Japanese "objectively" much harder than Spanish.