I've long been interested in translation – a consequence, I think, of English being my second language – and one of the best books about translation I’ve read is Mouse or Rat? by Umberto Eco (who has, of course, both translated and been translated).
To understand translation, it's important to realise that words have a semantic range, with core meanings and peripheral meanings (as well as connotations and associations). The ranges of neighbouring words adjoin, and possibly even overlap each other. Thus, for the colour “red” we have:
(image cropped from XKCD colour survey)
Core meaning: and similar colours
Other meanings: the hair colour called “red”
Associations: danger, communism
Neighbouring words: orange, brown, maroon, pink
Eco’s two example words in English are:
(photo by George Shuklin)
Core meaning: the house mouse, Mus musculus, and other members of the genus Mus
Other meanings: similar small rodents and rodent-like animals, a timid person, a computer mouse
Associations: timidity, inappropriate fear, domesticity
Neighbouring words: rat, fieldmouse, dormouse
(photo by “Kilessan”)
Core meaning: the black rat, Rattus rattus, the brown rat, Rattus norvegicus, and other members of the genus Rattus
Other meanings: similar medium-sized rodents and rodent-like animals, a scoundrel, a traitor, a person who frequents a given place (e.g. “mall rat”)
Associations: infestation, plague, unpleasant smells, treason, dishonesty
Neighbouring words: mouse
Now French (souris/rat) and German (maus/ratte) likewise have two words, but Latin (mus) and ordinary Italian (topo) make do with only one, which covers the combined semantic range.
What is to be done, then, in making an Italian translation of “How now! A rat?” (Hamlet, Act 3, Scene 4)? In German, we get “Wie? Was? Eine Ratte?”, but in Italian?
The usual (and, Eco thinks, best) Italian translation is “Cosa c’è? Un topo?”, even though this does not have all the connotations of the English “rat” – something has been lost in translation. However, in translating the French “rat” in Camus’s La Peste, some extra help is needed for the reader, because the plague-carrying properties of rats are particularly salient in that novel.
Similar issues arise in Biblical translation. While Greek has distinct words for “son” and “daughter,” for example, it only has one word (adelphos) for brother/sister/sibling. The ambiguity is mostly resolved by Greek’s gender-based noun endings, so that adelphē definitely means “sister,” adelphai definitely means “sisters,” but adelphoi could mean either “brothers” or “brothers and sisters” (although adelphoi kai adelphai would definitely mean the latter). There has been debate, therefore, on how to translate the frequent Pauline salutation adelphoi (“brothers” is the usual answer).
Conversely, the English word “love” corresponds to several Greek words. What is to be done, therefore, with a passage like John 21:15–17, where the two verbs agapaō and phileō are contrasted? Mouse, or rat?