Ah, we disagree my friend. I think languages can be easier or harder based on other criteria too, and not only familiarity.

Suppose an alien, the kind from outer space, crashes on earth and now needs to learn a language to communicate with humans.

It's not a stretch to consider that all human languages are so far removed from his own as to be considered equally hard to learn if looking only at familiarity. In this scenario, surely there are features of individual languages that make them harder to learn - stuff like gendered articles as mentioned before, as there's no logic to them and have to memorized.