I found (well, I was directed to) this intriguing graph showing the equivalents of “It’s all Greek to me” in thirty languages.
It is a thing of interesting revelations – such as Spanish (Spanish?) being the epitome of fogginess to Germans and Macedonians, and Javanese entirely baffling the French… I cannot say I ever heard a Frenchman or Frenchwoman refer to Javanese in this way – but then I am Italian, and I doubt I ever heard anyone use Aramaic as a synonym for unintelligible, either… Turkish, yes. Arabic, yes. Aramaic, never – and this in spite of having frequented a lot of theologians, a bunch of people more exposed to Aramaic than most.
Of course there can be local, regional, familiar or individual variations: my family’s favourite impenetrable language is Sanskrit, I know people who use Finnish in the same way, and it is not uncommon, in my corner of the world, to make jokes about the impenetrability of the dialect of Bergamo…
So perhaps the map is to be taken with the tiniest grain of salt – but is still very interesting as a map of stereotypical incomprehensibility.