Native speakers of the target language are best for translation work because they know the language and the culture from which it comes. Since they are a translator, they’ll also be fluent in English, or the language of your document, so will be able to understand it and then put it into the right words when they do the translation. Knowing the culture is very important, which is also why you should avoid computer generated translations, plus native speakers can understand connotation and denotation as well as slang and words that have other meanings which would not be appropriate.