"Unpristine" is very odd word, indeed, I doubt that it is a word in English. Although one can describe a mirror as being pristine, this would not be a typical description of a mirror. I gather that the German is a play on some existing German phrase to describe a clean mirror. Unfortunately, I don't think there is such a phrase in English, so there is no phrase that you can reverse.
How about "a cracked mirror." This is a pretty standard phrase in English to describe a mirror that no longer serves its purpose. Of course, you could just say "a dirty mirror."
Or perhaps "a dark mirror." There are over a million google hits on this phrase, although many of them relate to a computer game called "Dark Mirror." I would understand "dark mirror" as an allusion to the bibical quotation from St. Paul about "through a glass darkly." Indeed, there is a website with the name "Dark Mirror" and the author explains the title like this:
"The title ?The Dark Glass? is inspired by the Apostle Paul?s words in 1st Corinthians 13.12, where he states 'For now we see through a glass, darkly; but then face to face: now I know in part; but then shall I know even as also I am known.'" [ http://www.thedarkglass.net/page/2/