According to the Webster's 1913 accessible frim KDE thanks to kdict, one of the meanings for idiom is
the syntactical or structural form peculiar to any
I think that syntactical/structural forms are the way of capturing
in syntax recurrent programmatic patterns so they stand out.
So a language that captures patterns syntactically absent from other languages is certainly idiomatic. Perl is very aggressive in
capturing patterns with its syntax so it is certainly idiomatic.
A language can also be very idiomatic by its very repetitive syntax like Lisp and its "clipped nails". I would use the expression syntactically idiotic here to qualify Lisp.