Language is the most distinctive feature of humans, but there is no consensus on what is characteristic of language. From the point of view of computing, we argue that ‘the human brain circuitry that implements language is Turing complete’. This thesis makes evolutionary sense and natural languages are expressive enough, but two issues against it remain: natural language syntax is decidable and not every possible language can be a natural language. We answer the first by showing that the syntax of a complete language can be decidable, and by blaming functional semantics for the undecidability, where functional semantics is the semantics of syntax. To answer the second, we distinguish native language, first language, and later languages, where all natural languages are first languages acquired during a critical period that eases the process by preventing some possibilities. The thesis supports the weak version of the linguistic relativity hypothesis, and explains the rôle played by language in the cognitive gap that separates our species from the rest.