AFAIK all ML languages support tail call optimization, and put the emphasis on immutable data structures.

Nope, Scheme is one of the few languages that require tail call optimisation as per language standard.

There is no emphasis when mutability is one mut and ref cell away.

OCaml doesn't have a language standard afaik, and its implementation(s) support(s) TCO, so imho that's more important.

'Emphasis' just means the 'paved path', i.e. default data structures like records and variants are immutable, default list type is immutable. Of course if you want mutable stuff it's there, but it's not the default.

Even so, OCaml isn't all ML languages.

Afaik all ML languages have TCO, let me know if I'm wrong.