Whenever you translate, there is a kind of "semantic loss", which may be easily circumvented when it comes to fictional texts but which may endanger the essence of nonfictional texts such as philosophical treaties or academical research, since the latter kind of texts may contain certain key phrases whose connotations would get lost in the process of translation. If concepts of a certain general interest are concerned, the words signifying them are included into the language system that had no way of referring to those concepts beforehand. Blitzkrieg, schadenfreude, realpolitik and gedankenexperiment are examples for such transfers into the English language.
If the concept signified by a certain word is more specific, there's no need to transfer it beyond the very text it is used in. "Waldesruh" (literally "silence of the woods" or "silence within the woods") is a proper example. I don't think that this word has found its way into English, and yet it's impossible to really translate it, since "silence of the woods" doesn't convey the culturally related imagery (nor its allusions to romanticism or the implied pastoral atmosphere of peacefulness) of this very word.