But, for my own part, it was Greek to me

From Shakespeare’s Julius Caesar, 1601:

CASSIUS Did Cicero say any thing?
CASCA Ay, he spoke Greek.
CASSIUS To what effect?
CASCA Nay, an I tell you that, Ill ne’er look you i’ the
face again: but those that understood him smiled at
one another and shook their heads; but, for mine own
part, it was Greek to me.

Each country has a notional language that is symbolic of being especially incomprehensible. In England we have double Dutch. In fact Dutch is one of the easier languages to learn for English speakers and this position is bequeathed more for political and social than linguistic ones. Shakespeare is making use of the fact that the characters are Roman and, as far as the audience would expect, couldn’t (or wouldn’t want to) speak Greek. The phrase ‘it’s all Greek to me’ is now commonplace. Other languages which have exactly the same format as ‘It’s all Greek to me’, substituting their mutually incomprehensible language of choice are:

Dutch – ‘Dat is Latijn voor mij’ [That is Latin to me]
Italian – ‘Per me e arabo’ [That’s Arabic to me’]
French – ‘C’est du chinois pour moi’ [It’s Chinese to me’]
Esperanto – ‘Gi estas lau mi Volapukajo’ [It’s all Volapuk to me]

Volapuk was a 19th century invented language considered to be more obscure than Esperanto.

Other authors used a similar phrase to ‘It’s all Greek to me’ around the same time, so it can’t be said that Shakespeare coined it. There is also a medieval Latin phrase which translates as ‘It is Greek; it cannot be read’, which is earlier and could be said to be the origin of the term.