@Giorgio: Really, you should have defined both whistleblower and Nestbeschmutzer in your first posting here, with examples of both to show that they're not equivalent. If you care so much about this word, you should still do this research, which is the job of the person making any suggestion in New or Wrong Entry.
(The Wortschatz Uni-Leipzig
is a good place to start; see the international portal farther down the page for English words. Or use a search engine and limit the search by specifying a newspaper, magazine, or TV website, or using something like google.news.)
That said, I agree that the existing entry is wrong. In fact, I tend to agree with Giorgio on almost everything except his suggestions for translation -- but that's a big exception. We can't just make up new words. If there's no exact equivalent, a choice of several roughly similar existing words and/or descriptive paraphrases may be the best we can do.
As for the English, I've copied definitions from several published dictionaries for the sake of completeness, but I must say I think only the one in American Heritage is correct. The others aren't specific enough; they could just as easily refer to any police informant, even an ordinary citizen who reports a neighbor to the police.
The word 'whistleblower' really does mean specifically an informant from within an organization,
which in practice is nearly always a workplace, as the web definitions above confirm. So I've offered a couple of draft suggestions above, in the hope that German speakers will refine them. I don't know, for example, which is the best word for 'employee' or if it would be better to give more than one choice, like der/die Angestellte and Mitarbeiter/-in.
I'm also with AHD and Giorgio for 'whistleblower' as first choice for AE spelling. But whichever spelling LEO decides to put first, both should definitely be listed -- and maybe even also 'whistle blower' as two words, though IMO that's really a distant last choice.
As for the German side, I agree that Nestbeschmutzer needs a new translation, and preferably more than one. But 'nest-fouler' or 'nest-soiler' isn't a normal English word either, and it fails to explain the figurative sense (as does the lame Oxford-Duden definition). I don't like Pons's 'runner-down' at all as a noun, and 'denigrator' not much more. But it may be the best bad choice; something like '(person who runs down / denigrates his/her own family / country / etc.)' could be clumsy as a dictionary entry, though better as a translation.
The closest I've been able to think of in just one word so far is 'ingrate'; I realize it's not exactly the same, but it might be worth listing as one of several options.
Apparently Nestbeschmutzer isn't limited to the workplace; possibly it's not even commonly used in that context at all. But it would be good if German speakers could confirm that and give definitions and examples of how it is
used. More examples might help English speakers think of a better translation. Again, that should be at least partly up to Giorgio.