English, a language that looks much like our own English language. Certain words of zombie-English would have to have meanings somewhat different from their English counterparts. I use this example because it is so familiar from Searle's Chinese Room argument. To understand Chinese is more than to be able to produce passable answers to questions. There is something it is like to understand Chinese, or anything else.
Understanding has a phenomenology.
For a zombie, however, it is not like anything to understand Chinese, or anything else, because it is not like anything to do anything. We can imagine being visited by the beings from zombie-Earth. Our natural science might look a great deal like theirs, and our mathematics would likely be very similar. But there would also be some interesting differences, especially in the domain of philosophy. Consider, for example, the phenomenon of dreaming.