Should they be a native speaker?

As a rule, your translator should be a native speaker of the target language (the language you’re having the document translated into). This is because it is much easier to write grammatically correct sentences that mean what you meant them to mean in your native language.
Have you ever read a manual where the sentences didn’t quite make sense and you were left guessing what the instructions meant? Or what about a menu offering DUMPLONG STUFFED WITH SIRLOINS GROUND BEET, ONIONS AND SPICES? These are examples of what can happen if the translator is a native speaker of the source language (the language of the original document) rather than of the target language. It can get even worse when the translator isn’t a native speaker of either language.
However, not all non-native speakers write foreign languages that badly. Some write them extremely well, especially if they have lived in the country for years (although number of years in a country is no guarantee of native-like language abilities). If they are a native speaker of the source language, you will also have the advantage that they will have understood the source document as a native speaker. For this reason, I would use the following rules for choosing which language the translator should be a native speaker of:

Do I need a specialist?