In English, one would say "but I'm not sure which/what kind" or "I think this is some kind of pie, but I'm not sure"
I am guessing the french version suggests the first translation?
I think in English it should be: it's a kind of pie, but i don't know what kind. Two kids are in a house looking at left over food: one asks the other, what is that? The other might say, it's a kind of pie, but I don't know what kind, it could be pineapple, mincemeat or rubbarb!