Der Spiegel posted a little quiz for people to test their colloquial English skills. Some of the questions strike me, as a native English speaker, as somewhat off. For instance, the first question is:
Sie kennen einen Geschäftspartner aus dem Privatleben und machen Ihre Kollegen darauf aufmerksam. Wie sagen Sie es – ohne unfreiwillig Gerüchte über Ihr Intimleben zu streuen? [You know a business associate from your private life, and want to mention this to a colleague. How do you say it — without unintentionally arousing scurrilous rumours about yourself.]
The second one is obviously anglicised German. The third sounds like you’re saying, I’ve actually met him, rather than knowing him by reputation or having heard him give a talk. The first one sounds like something I might say, even if in reality I’d be more likely to say something slightly more specific about the context from which I know him: He’s my neighbour, I know him from the rabbit-breeding club, we do hang-gliding together, etc. But their favoured answer is #3, and about #1 they have this to say:
TMI – too much information. Da hätten Sie auch gleich ausplaudern können, dass Sie die Person schon mal nackt gesehen haben. Ihre achtlose Bemerkung klingt auf jeden Fall so, als wollten Sie ein wenig mit einem intimen Geheimnis prahlen. Doch das will niemand wissen. Jemanden privat zu kennen, bedeutet im Englischen, sie/ihn in einer vertraulichen Weise zu kennen, die in der Öffentlichkeit nichts zu suchen hat. Nur als Tipp: “Private parts” im Englischen sind die Geschlechtsteile. Sagen Sie deshalb “I know him personally”, und Sie werden garantiert nicht missverstanden.
You might as well have blurted out, that you’ve seen this person naked.* Your careless comment certainly sounds, in any case, as though you wanted to boast of an intimate secret. But no one wants to hear this. To know someone privately means, in English, to know him or her in a confidential way that has no place in public discussion. A tip: “Private parts” in English are the sex organs.
*Which, in a German context, actually doesn’t necessarily mean that you know him well, but only that you’ve been to the same beach, or possibly the naked swimming hours at the local pool.
Common Infirmities 
I saw the game quiz and thought much of it was iffy. I knew the quiz too, but not in the biblical sense, and certainly not privately.