Grimm also use French as well! I was in a french lesson and almost had a heart attack when I found out a bit of trivia that ‘Renard’ means fox! Which is quite fitting to the character haha:) But yeah I speak a bit of german and they do completely fuck it up lol

Yes! The Grimm writers were probably thinking of the stories of Reynard the Fox in particular, because Reynard is one of the names Kelly mistakenly gives to him. 

There are many languages used in Grimm for the Wesen names, mostly German or French due the origin of most of the fairytale stories used, but any Wesen that originate from a particular culture tend to have their names in that language, like the Koschei, named after a particular Russian character.

Apparently the actual German spoken by characters improves when the guy who plays Meisner joined the cast because he speaks the language, and the French spoken has always been of high quality because Sasha Roiz (Renard) speaks French as well as Russian.