I may be wrong but I think that's not the view of the modern PL community. If your API talks about values, combinators, and has (implicit) laws about them then I would certainly consider that a language.
Since object orientation brought the possibility of adding little languages to the masses and a few decades later value objects and lambdas and generics where added to all OO languages, a postmodern view may be quite an applicable and useful perspective to deal with this question.