The word tzedakah is untranslatable because it joins together two concepts that in other languages are opposites, namely charity and justice. Suppose, for example, that I give someone £100. Either he is entitled to it, or he is not. If he is, then my act is a form of justice. If he is not, it is an act of charity. In English (as with the Latin terms caritas and iustitia) a gesture of charity cannot be an act of justice, nor can an act of justice be described as charity. Tzedakah means both.
Rabbi Jonathan Sacks, To Heal a Fractured World: The Ethics of Responsibility (New York: Shocken Books, 2005), 32.