My mothertongue is Spanish. And in Spanish-speaking countries, when something is difficult to understand –like maths– we say “it sounds like Chinese”.
But have you not wondered what the equivalent in other countries and languages is? I know that in English it sounds like Greek.
It’s funny how there’s a country for everyone.
There have been studies on the matter; there’s a Wikipedia article on the topic “Greek to me” with a table of correspondences and several other discussion threads as well.
Behold this image below which I got from a post written a few years ago in the LanguageLog blog.
Linguistic Mark Liberman produced what he called a “directed graph of stereotypical incomprehensibility”:
I want to emphasize the Czech, Macedonians, Croatians and Germans since they refer to Spanish when they understand nothing of something; the Japanese seem to understand everything and haven’t the problem the rest of the world has.
And the Chinese. The Chinese say “It is Heavenly Script.”
[EDIT] I’ve edited the diagram. When Japanese don’t understand something, it also sounds like Chinese. I’ve added Denmark.
When Danes don’t understand something, it sounds like Volapük to them.
According to /u/nuephelkystikon, Germans, if they don’t understand a technical term, say it’s Fachchinesisch “technical Chinese”. If they don’t understand a geographical term, it’s Böhmische Dörfer “Bohemian villages”. And if they don’t understand a cultural custom, it kommt mir spanisch vor “appears Spanish to them”. There is also the general expression Kauderwelsch “drooling French/Celtic”.