Well, to start, the sound of it has to be convincing. And then also, the actors have to, not necessarily believe it but they have to get the feeling of it as if they speak this language very well. Part of that is the performance, and part of that is the construction: in order for it to be a believable language it has to have a believable intonational pattern, it has to work grammatically even if that's not something that anybody can tell just by listening to it. Going further, there’s also consistency so that one word means the same thing from line to line. And even beyond that is functionality: does it have a full and complete grammar, does it have all the words to convey any possible meaning?
That’s absolutely right, functionality is a bit of a low bar to clear because beyond that, there is of course originality. As you mentioned, you could just rework the English language and it would be both consistent and functional but it wouldn't be original. So then a step beyond originality is authenticity. In other words, does this thing make sense as something that might have evolved for the speakers in the show? Not just does it fit the culture but could it actually pass a test of being a natural language?