No one has mentioned the old Italian pun, "traductore, tradittore" (translator, traitor). I recall a comment by an interpreter at the UN on interpreting a speech by Kruschev. When he told a traditional Russian story to illustrate a point, the interpreter realized that the story would make no sense to English hearers, so he quickly realized this and substituted a completely different story which would be familiar and make the same point.
Research has shown that, even at a literal level, different languages "pack" different kinds of information in, for example, verbs. Both English and Spanish have a general verb of motion, "go"/"ir", but Spanish has a verb "salir", which would be translated by the Verb + Particle combination "go out", which separates the motion and direction. Translations of novels from English to Spanish and vice-versa show differences in the amount of information omitted or added.
Even in technical translation, people working in a field have standard expressions which a translator unfamiliar with the field would be unfamiliar with. A professional translator hired by an archeology journal to translate article summaries into Spanish produced odd literal translations because she was unfamiliar with the field. Literary translation, of course, is vastly more complex because it includes a great deal of cultural and emotional content, and poetry in particular requires a complete re-casting. I have heard it said that the English translation of the Rubaiyat of Omar Khayyam is better than the original.
Pedagogically, translation provides an important "boot-strapping" which speeds access into the target language. Learning a second language in a school setting can be likened to gaining familiarity with another country and its culture by looking at pictures in a book, or reading descriptions. Only by gradual steps does one gain greater sociolinguistic and cultural knowledge, but only living in the other country gives one the experiential and existential sensibility for deeper knowledge. (Even then, of course, no one can ever become a complete native of a second culture.) All second language learners necessarily begin with translation, overt or covert, until their linguistic competence reaches the point at which they can use the L2 without conscious awareness, but even then, psycholinguistic research shows that the L1 is being activated, suggesting that translation is still taking place subliminally.
University of Arizona
Tucson, Arizona, USA