To a new and experimental language it would be a small change. Try adding a built-in map to C or Java. Lambda expressions took year to add to Java for example.

Now if say Nimrod or Dart acquired built-in sets (I don't know maybe it already does) it wouldn't be too remarkable. They are pretty experimental.

Also there is a difference between having a library and a language built-in. All those languages including Erlang, have libraries that provide associative data structures.