So where can you find out about legal terms? Here are some of my favorite places/reference books. This is not intended to be an exhaustive list, just a place to get started.
A good general reference work is Black’s Law Dictionary. You can buy a new one at any major bookstore (or, for that matter, many minor ones that sell new books). Because this book is so widely used, you may be able to find a used copy in places that sell used books. While I was typing this up, for example, I checked www.ebay.com and found 27 copies of various editions for sale by various people at prices ranging from $1.00 to $60.00. You don’t necessarily need the latest edition. If you need the latest case law on, say, estoppel, you should probably be consulting a lawyer. (See the note/disclaimer section about how to find one.) Two or three editions back is quite okay.
http://www.findlaw.com will get you a bunch of general legal information. I recommend setting up an account for yourself on the "legal professional" part of the site. "Legal professional" includes some categories for non-lawyers, so don’t worry about using it. It has links to codes and case law (on the "legal professional" side), which can be useful if your source text says "as this term in used in [some provision of law]".
Http://www.lectlaw.com, the ‘Lectric Law Library, is a very well-researched site. If you’ve never used it before, I recommend you take the virtual tour. Part of it is the ‘Lectric Law Lexicon, definitely full of lots of definitions.
http://www.courtstatistics.org/ is the link to the Court Statistics Project. The "Court Structure" link gets you to the various court structures in the various states. Names of courts are not necessarily intuitive. For example, New York has multiple Supreme Courts and their verdicts can be appealed within New York.
Locate the relevant country and see if the link takes you anywhere you’re interested in going.
The World Legal Information Institute, http://www.worldlii.org, gets you into lots of places. You can locate the relevant country and go from there. Or you can use their search engine and find material on the subject of interest to you.
http://www.saccourt.ca.gov/general/legal-glossaries/legal-glossaries.aspx is a site put up by the Superior Court of California, a state that has lots of people speaking lots of languages. In some cases, the glossaries have rather long explanations. For example, the English-Russian translation of "abduction" is "Преступление, заключающееся в уводе жены, ребенка или подопечного путем обманного убеждения или применения силы," which is a good explanation but slightly long for simultaneous interpreting.
The European Union translates lots of material into the official languages of its various member countries. At http://europa.eu/geninfo/atoz/en/index_1_en.htm , you can find an index to the various things it writes about and, once there, find documents about the subject in question.
Some specific languages:
For any of these dictionaries (or for any bilingual dictionary in any field), if you don’t understand the target term, you should at least run it through a search engine to see if it’s used the same way. I’ve often seen references to "resolution of a contract" as a translation of "resolución de un contrato" in translations done by people who don’t realize that "resolution" isn’t used this way in English. (A more usual translation is rescission or revocation.)
Creifeldt: Rechtswörterbuch. A good monolingual German dictionary on law.
Dietl/Lorenz: Wörterbuch für Recht, Wirtschaft und Politik, probably the best-known English-German and German-English legal dictionary around. It can be obtained in a print version or a CD Rom.
http://www.simone.it/codici/index.htm is provided for personal use by the Edizione Simone, which also publishes print versions of these codes. The thing I like about these is that they have links on terms within the text that lead to definitions of the terms.
Kalina-Prasznic, Urszula: Encyklopedia Prawa, a good monolingual Polish legal dictionary
DeBeer, Shane: Dictionary of Business / Legal Terms, has a lot of terms without much explanation.
http://www.sokr.ru is an abbreviations site. It gets updated often by users and one of the first things one sees when one goes there is a list of new abbreviations added.
Alcaraz Varo, Diccionario de términos jurídicos, good general legal dictionary
Becerra, Diccionario de términología jurídica mexicana, very strong on Mexican terms and probably the best if you deal with Mexico often
Cabanellas de las Cuevas and Hoague, Diccionario jurídico, a two-volume set (often sold as Butterworth’s). Well-researched, although many of the entries are explanations that can be difficult to plug into a translation. For example, the entry for "cancelling date" is "fecha en que se procede a resolver un contrato o hasta la cual tal resolución es posible". That’s a bit longer than the English, though it describes the concept well.
Mazzucco and Maranchello, Diccionario bilingüe de terminología jurídica. Comes with CD Rom. Great for use if your material comes from Argentina (or even if it doesn’t). Patricia Mazzucco, one of the authors, is probably one of the best-known legal translators there.
West, Thomas L., III, Spanish-English Dictionary of Law and Business. Well-regarded and well-researched.