There's no such thing as an untranslatable concept. We all have a human brain architecture after all. But if you're doing a translation at all, you're probably translating sentences/paragraphs/dialogs where you can't just insert an aside every time a word without an exact equivalent comes up. Especially if it is a word that gets used many times, not having a brief equivalent is a problem. In that sense, it is untranslatable.
Also arises when a word is loaded with lots of extra cultural information that the short couple of sentences explanation doesn't capture. Like, you can probably find a good local equivalent of "witch" in many languages. But, it probably doesn't include living in a house made of candy, wearing pointy hats, having warts and green skin, flying on broomsticks, and forming covens of 3 or 13.