For all that det = it and she = hun, that does not address the point that "Is it an unfamiliar girl?" is unidiomatic as a translation of "Er det en velkendt pige?" -- except in very restricted (not to say, contrived) circumstances (perhaps where "it" stands for something like "the situation" rather than the girl herself). The fact is that we are asked to provide an English translation, not a word-for-word breakdown. Otherwise we'd have to translate "Jeg kan godt lide at synge på tysk" as "I can well suffer to sing on German"....
"Er det en velkendt pige" is strange i danish aswell as in english. and i do not tnink that i would ever use it other than in contrived situations. It is a valid sentence but not something you normally say. (like "the bear is raising the girl" and some of the other stuff on danish duolingo)
I would never translate "is she a familiar girl" to "er det en velkendt pige" the standart translation would be "Er hun en velkendt pige"
"Er det en velkendt pige" implies that you do not know if you are talking about af female or it could be "is that a familiar girl"
To me "en" refers to "pige" and "velkendt" refers to "det", it's the only thing that makes sense with the limited knowledge I have but I don't know why it happens. I'm learning this language too so I might be wrong as well. Maybe a native speaker or someone who owns the language can clarify it for all of us