Both German and French "r" have a wide range of realizations. In extreme-"refined" speech, I think both are uvular trills (http://en.wikipedia.org/wiki/Uvular_trill), and therefore virtually the same. Personally, I produce more of a trill in German, and more of a fricative in French (http://en.wikipedia.org/wiki/Voiced_uvular_fricative), which may sometimes get unvoiced (especially when it's at the end of a syllable as in "garçon" or "centre") and will sound sorta like the German ach-laut as in Bach. Hope this helps! (:

I don't know the rules, but by practice I noticed that it’s quite the same as French in some words like: “Reden”, ”Raum"... It's the soft R and like the Arabic "KHA" خ , the French R in "train" " Prier" or even the German "CH " like in " Buch" " Bauch" ... in words like : " Sprache" " I'm not sure 100%, a professional’s opinion is still needed