It just hit me what would Greeks say in the situation the English speakers say “It’s Greek to me.”
http://en.wikipedia.org/wiki/Greek_to_me
Chinese is most referred to by the alphabet-using people. But then what would a Chinese say?
In Korean wikipedia it is stated that they say : 跟天書一樣 [跟天书一样] (gēn ti?nshū yíyàng) (It looks like a book of the Heaven.)
Interesting. In Dutch they something like; There was not a word of French in that. This is of course when it is not Greek to me.
Italians say: “A me arabo!” “It’s Arabic to me!”
On an (almost) totally different subject: When that made-for-TV movie “Citizen X” came out, about the Russian serial killer, it started me wondering, what letter would Russians use to indicate “the unknown” as in “X-rays” or “the X factor?” There is no Cyrillic letter that is phonetically “ks.” I have since read somewhere that they use the letter X “kha,” presumably because it looks like the Roman X. So the title of the movie would be “Гражданин Ð¥.” Can any of our Russian (and/or Ukrainian) friends confirm this?
Yes, we go for Chinese “Είναι Κινέζικα” to be exact 
And it’s one English expression (it’s Greek to me) that I always try to steer clear of (and not to blame Shakespeare)
You may find this thread interesting: All languages" It is Greek to me
Arvid totally unrelated indeed. In MG by the way , “X factor” in maths is “άγνωστος Χ”. Bear in mind that in modern Greek Chi is pronounced like a strong “H” or maybe “ch” in loch
We have “It seems/songs Chinese (to me)!” (Spanish, Me suena a chino) in Spanish.
What the..? The Germans say: Das kommt mir spanisch vor! Of all the weird and confusing languages in the world, they pick Spanish? 
Pero si es de lo más fácil…!
As for Koreans, there’s no idiomized expression for that. We often say “한국?? 너무 어려워Korean’s too difficult(to understand)”, sometimes mimicking a foreigner speaking Korean, “항쿵마룬 노무 오료워요Hang-kung-marun nomu oryowoyo,” because many a people has bad practice of speaking non-sentences, syntax mismatches, and ambiguous expression of ideas, etc. And some translations of technical documents are done so carelessly that it’s better to learn English and read the original document.
In the internet era, young children developed a way to write Korean with replacement characters that resemble Korean alphabet or to severely distort the orthography and they call it “Alien(extraterrestrial) Language” but that’s not a new kind of language, but broken and distorted writing practice. And it didn’t become an idom for non-understandable things said.
Well, occasionally we might ask, “그거 어? 나? ??야?What language(lit. which country’s language) is it?” when we face an un-understandable sentence(with many math, sceience, or religious jargons). But we don’t specify a language. But this could be the closest equivalent.
It could possibly be due to the fast speech of Spanish speakers. (Spanish songs often have several syllables for a single note.
) If not for that, Spanish would have no reason to be regarded a “difficult” language.
As for Chinese, although the characters are infinitely complicated, I found the spoken language not that difficult, especially to learn by listen-and-repeat method.