The necessity of performing an English translation of Balmont's text (as opposed to reverting to Poe's original) can be easily explained: given that Rachmaninoff's setting of Balmont is just as free as Balmont's translation of Poe, Poe's original text is highly incongruous with Rachmaninoff's musical setting.
The necessity for an English translation of Balmont's work (as opposed to returning to Poe's original) can be clearly explained: since Rachmaninoff's composition based on Balmont is equally as creative and expressive as Balmont's own translation of Poe, Poe's original text does not fit with Rachmaninoff's musical interpretation.