Different languages use different ways to describe the same object. This is one of the fundamentals posited by Ferdinand de Saussure. What Poles call grzyb, English speakers call mushroom. Two different sounds for the same object.
The process of translation becomes interesting when there is more to the object than a simple word. Let us take, for example, the English word capital to mean the ‘head city of a country’. The word derives from the Latin capitalis (of the head) from caput (head). In Polish, however, we have stolica. We can also talk about capital punishment in English (i.e. off with the head) which in Polish is simply kara śmierci.
The more knowledge the translator has about a word, the more accurate the translation. In the above example, we have a metaphorical extension of the Latin word caput which has then been borrowed into English whereas in Polish we do not. The reverse could also be true where a word has metaphorical roots in Polish but not in English. A wonderful example is the Polish (słone) paluszki which might roughly be translated as salty sticks (never fingers!).
Body parts commonly lend themselves to metaphorical use, but again, it is not the simplest words that make for the more interesting work for the translator but rather the more complicated. Metaphors and tranferrals of meaning are not straightforward and require a little more thought. For instance, in English we speak of a plastic sleeve used in the office. The inexperienced translator may render this as plastikowy rękaw. The correct equivalent would, of course, be plastikowa koszulka which, in turn, could not be translated into English as plastic shirt.