Further to Wyatt Mason’s quoting Vladimir Nabokov’s three evils of bad translation — ignorance, omission, and adaptation — here is the self same translator of Verses and Versions: Three Centuries of Russian Poetry (Harcourt, 2008), on the requirements a translator must possess in order to render an ideal version of a foreign masterpiece:
First of all he must have as much talent, or at least the same kind of talent, as the author he chooses. In this, though only in this, respect Baudelaire and Poe or Joukovsky and Schiller made ideal playmates. Second, he must know thoroughly the two nations and the two languages involved and be perfectly acquainted with all details relating to his author’s manner and methods; also, with the social background of words, their fashions, history and period associations. This leads to the third point: while having genius and knowledge he must possess the gift of mimicry and be able to act, as it were, the real author’s part by impersonating his tricks of demeanor and speech, his ways and his mind, with the utmost degree of verisimilitude.