Many languages have a phrase corresponding to It’s Greek to me to signify that something is incomprehensible or makes no sense to the utterer/writer. The English version may have started in the Middle Ages as a translation of the Latin phrase, Graecum est, non legitur ‘It’s Greek, [hence] not readable’, at a time when knowledge of Greek among scribes was on the wane.
When it comes to other languages (Arabic, French, Hebrew, Russian, among others) however, it is Chinese that is most commonly  referred to, and what is meant specifically is the writing system rather than the spoken language. This is confirmed by the Japanese version, sanbun kanbun ‘gibberish’,” where the literal meaning of the two components is ‘prose’ (sanbun) + ‘Chinese script’ (kanbun). Russian kitajskaja gramota (китайская грамота) ‘Chinese charter/alphabet’ also makes explicit reference to the script.
All the Slavic languages have in fact incorporated what can be interpreted as the ultimate degree of unintelligibility of speech by likening the speakers of one foreign language in particular––German––to those who cannot speak at all, namely mutes: R nemeckij [jazyk] (немецкий язык) ‘German [language]’, etc., takes its formal and semantic designation from the Common Slavic adjectival base nem– ‘mute’.
In English, when we want to single out speech or writing as crabbed, miscegenated, or full of incomprehensible words––and, therefore, evaluated as a degraded form of language–– we typically resort to words like jargon, lingo, pidgin, patois, and argot; or to compounds utilizing the suffix –ese, as in bureaucratese, legalese, etc.––doubtless derived from an extension of the suffix in Chinese.
Speaking of jargon (which is probably of French––at any rate, of Romance––provenience), it is interesting to note that in pre-revolutionary Russian (the language of my parents), the word жаргóн also was in common use to mean Yiddish, specifically by Jews themselves. Speakers of Yiddish evidently felt no pejorative taint in resorting to a label in Russian that reflects their rich mother tongue’s hybrid (German, Hebrew, Slavic) grammatico-lexical makeup.

MICHAEL SHAPIRO