I think there is a difference between classic authors whose writing is admired and do travel down generations, but which few people read. Chaucer, Milton, Scott and classic authors whose writing may be of no greatmerit but who write books that people enjoy reading.
I have seen a list recently, that is based on UNESCO statistics and liste the top 40 authors by the number of different languages they have been translated, which is a good guide to universal appeal.
In order of number of languages they have been translated into Agatha Christie heads the list, 2< Jules Verne, 3< Enid Blyton, 4<William Shakespear, 5< Barbara Cartland, 6< Danielle Steele, 7<Hans Christian Anderson, 8< Steven King, 9< Grimm Brothers, 10< Mark Twain.
TwiceAsNice neither Chandler, Maugham or Greene feature on the list.
I think we need to be clear what we mean by a classic. To me it doesn't necessarily mean a book of recognised literary merit, with a small, but persistent following. It means a book that is widely read (including in other countries) continually down several generations, where most people are likely to have heard of the writer and have easy access to a copy of the book.
I think the authors listed by number of languages they are translated into is as good a start as any.