A tricky challenge for every translator is how to deal with idioms. If you translate them verbatim, they will probably make no sense in the target language and if you choose a neutral expression, it may change the style or the atmosphere the original text seeks to convey. Regardless of the difficulties idioms present, however, they are also any language’s treasure trove because they tell you so much about the people, their ways of thinking and their ways of life.
Let’s take the following German expression:
Dastehen wie der Ochse vor dem Berg / wie die Kuh vor dem (neuen) Scheunentor (literally: to stand there like an ox before a mountain / like a cow before the (new) barn door)
Meaning: you are at a complete loss 
When oxen and cows are faced with an obstacle in their way (or even the new appearance of an old obstacle, such as a new door), they apparently don’t know what to do and simply stop before it.
I have seen it translated as “like a (dying) duck in a thunderstorm,” but that does not quite express the same thing. The “duck in a thunderstorm” conveys a sense of being hopeless, crestfallen, or dejected. While it implies helplessness like the German expression, the mood is entirely different: The ox or cow is unperturbed, not miserable. It simply waits for guidance.
The English expression is derived from a folk belief, which held that the noise of thunder particularly affected ducks, so that one had to protect the eggs from the sound or no ducklings would hatch. By the same token, it was thought that hatched ducklings could easily die during a thunderstorm. 
Because their deaths was preceded by much eye-rolling, which gave them a somewhat surprised look, the expression initially described a stunned or startled expression. Yet, over time, the meaning changed. Today, looking “like a duck in a thunderstorm” stands for being forlorn or sorry for oneself.
(Cf.: Minard, Antone. Western Folklore, Vol. 69, No. 1, Winter 2010, found here)
(Eine deutsche Erörterung des Themas befindet sich hier)