在這句中，Greek為何是專有名詞？

In this sentence, the word "Greek" in deed refers to the "Greek language", so it is in fact a proper name and needs to be capitalized.
The meaning of the sentence is that my mother does not know anything about computers, and does not know the Greek language, so to her, using a computer is just like reading the Greek language.

Greek is a term which is similar to idiom.
Greek is quite too old or far away for us to know
so 'The computer is Greek to my mother.'
means that ''My mother knew nothing about the computer or
The computer came as a conundrum to my mother

Whether Greek or not,It's all Greek to me is an idiom.It's an informal saying that she cannot understand it.
eg:-She tried to explain how the computer works, but it's all Greek to my mother.
Answer:-Idiom;It's all Greek to her.