Do I need a proofreader?

The gold standard in translation is that every document should be translated by one person (the translator) and then checked by a different person (the proofreader).
The reason having a translator and a proofreader is the gold standard is because nobody’s perfect. Sometimes a translator will translate a document perfectly first time, but sometimes they won’t, because it’s very easy to misread or overlook things, particularly in longer sentences and particularly in language combinations where the word order in one language is very different from another. The reason that a second person is used is because of how hard it is to correct your own work – because of the way the brain works you are likely to overlook a mistake you just made the next time you look at a document.
I would use the following questions to decide whether or not you need a proofreader:

How much should I expect to pay?