Based on my surely not-that-accurate restaurant-menu analogy post of the other day, which at least highlights some of the main characteristics of legal translation, I think we can say that to translate legal documents you need to:
Keep up-to-date with changes in your legal systems. Owing to the changeable nature of them.
Have a good background knowledge of your legal systems. So you have a framework for understanding changes and new information, and, given the complex nature of the field, can understand what’s going on in the first place.
Remembering, of course, that you need to know about both systems because, depending on how you became a translator, you probably know one system better than the other, which may not correlate with your native or target language.
Be creative and a little bold when translating non-equivalent terms. Given the non-equivalent and changeable nature of legal systems, other people’s resources will only take you so far. Don’t look at translations, even from respected sources, as set in stone.
Know how legal documents are written in the target language. So you can make the translation as natural as possible — probably not that easy given the possible general perception that legal translation is often bad. This is about language and form (the second of which didn’t turn out to be too important for menu translation, but I’m sure that wasn’t the only flaw in the analogy).
Understand how documents are used in different legal systems and whether any differences are important. This is related to customs; some documents have non-equivalent purposes and functions that need to be taken into account.