Language. You can't communicate without it. Sign language is still a language. Body language is too. But how do you teach a person who knows not a single language the first language? To be honest, I don't know, and I don't think we'll ever know. However, that's not to say that the process of learning a language can't be sped up. The human brain is very capable of stuff.
We learn our first language by getting immersed into it. Our parents speak to us, and that's how we slowly learn to say the first word, typically mother or father (related. I know it's highly unlikely that the first word uttered by anyone is that advanced.)
I'm not sure if other countries have generally different practices, but here in Singapore, the elders tend to teach infants or toddlers bad language (bad, not vulgar. I know what some of you are thinking...) to begin with. "This thing hot hot... don't touch k? Wait you burn yourself." Prime example I'd say. (it's a local example. I'm sure other regions in the world would have their own problems as well) We don't even hear adolescents speaking to one another this way! Don't forget that as young children, they'd learn by first mimicking. By teaching them such bad language, one would be wasting effort and opportunity, not to mention that later as they grow up, they'd have to unlearn the bad habits, which wastes yet more time and effort.
So, what would I propose? Start by speaking in proper full sentences, or at least nouns and verbs before adding adjectives, conjunctions, and all the other stuff. This is for english, but you get the idea. Start simple, but make sure that it all can be built upon, and nothing has to be unlearned.
I don't know if this is possible, but if you start off with full sentences of words with four syllables or more, and etc. I wouldn't be surprised if they start getting the hang of the language much faster than others.
After learning how to speak properly, I'd recommend then widening vocabulary. This cannot, and should not be done through memorisation. That totally kills the fun, and would make it a chore to learn. Instead, try incorporating them into regular conversations. Things that are used often will tend to stay in memory.
So once communication is up, learning would be much easier for the child. And one thing must be remembered. Learning should not be enforced, it should be encouraged. Next up... logical reasoning.