This is a nice article. I agree that natural language is the hallmark of intelligence, but intelligence stands completely apart from language in a computational neuroscience sense. There is good evidence that natural language is simply a communication tool evolved quickly, and uniquely, the human species. It is another tool used by your whole neural system to interact with reality, and one specialized to interacting with the reality of other members of your species. But, uniquely, human language is the most direct and comprehensive tool for communication directly between two neocortices, two brains. One brain communicating directly with another, needing neither hands, nor even speech. Language reveals intelligence. It is not its hallmark, I think. The intelligence per se lies dominantly throughout the greater nervous system. To see the intelligence in operation, in vision, for example, look to how language is structured.
The natural evolution argument says intelligence came before, long long before, natural language. And the natural language evolved in an evolutionary second, for only one species because the computations were largely already there. It was evolved explicitly for speech as the physical communication vehicle. We can now see how much of this fits together. So, yes, the Turing Test is best. But not because it tests language, but because it tests intelligence. Natural language is a modality. Just like vision, audition, making sounds, moving your body, sensing pain, etc. And it is structured in episodic hierarchical expression (both for perception and action), just like any other modality, computationally. But intelligence is not.
That computational similarity to how the brain uses other modalities is how language could come to be so quickly in the last ~250,000 years (of a billion years of life evolution), and specially, for Homo sapiens.
I think I might say instead of "hallmark" that language communication provides the most perfect test of machine intelligence if we are looking for intelligence like our own. This is because our language was evolved naturally over a billion years of trying to be exactly that. (I'm, of course, Speaking as a particular species here). That's why the Turing test is the right test. I believe you can find in hallmark of intelligence in a worm, too. It's fundamental to any neural system in any Metazoan creature.
