if you would say it after a conversation people would understand what you mean and might not even question whether that expression exists or not. But I personally would not say that. However I know people else have said: "Jetzt sprechen wir eine Sprache" / "Now we talk one language". So I guess it might depend a bit on where you are from etc. So.... I reckon.... with the right context, you could use it as an expression.
as an english idiom, i'd say 'now you're speaking my language' is more often used to mean 'i like/am very interested in the thing you just started talking about', no? at least that's what i think of it. maybe a british english thing (seems like something ray winstone would say?). any correspondence to the german there?