I was reading my copy of Don Quixote and came across the phrase "for they understood about as much of [his words] as if he had been talking Greek" in reference to some confusing stuff Don Quixote said. This book was originally written in 1605 in Spanish and uses Greek as the language that is typical of not being understood. In contemporary times we say things like "It's all Greek to me" to indicate that we don't understand something. Anyhow, why is it Greek that's commonly the language in this turn of phrase and how did this originate?
It's quite likely that the origin of the phrase comes from Latin "Graecum est, non legitur". Greek was a language of scholars much like Latin was, but unlike Latin it uses different alphabet and it's not related to most other indo-european languages. While it was expected of a highly-educated man to know Greek, many didn't.
In many languages such incomprehensible language is Chinese (including Greek), sometimes Spanish in slavic languages, to underline distance.
I'm not qualified to answer this but in Contemporary spanish that is not really used, me being a Spanish native speaker, Don quijote probably "said that" because of the alphabet