I'd like you to translate this letter into English by tomorrow.
Literal
Tomorrow [by-までに] this letter [object-を] English [to-に] fix-have-receive want.
直してもらいたい — 'want to have (someone) fix/convert' — layers ~てもらう (have someone do) with ~たい (want). 英文に直す means 'to convert into English writing.' The combination creates a polite but assertive request.