When translating legal documents from French (or indeed most other languages), it is important to be aware of the differences between English law and the law of the country where the document originates. Differences in legal philosophy, practice and system, as well as terminology and set formulas can vary considerably from country to country and legal system to legal system, and English language and law often have no terms or concepts that are exact equivalents of those in the source language. Under these circumstances, a translation can convey the overall sense of the text, but not necessarily the specific meaning of the original.
Usually a translator can find English terms that are a close enough match to the terms used in the source language. If not, the concept might be expressed by using more explanatory phrases. Very occasionally, it might be necessary to write a footnote. However, no translator can act as a lawyer or legal adviser and clients should therefore seek proper legal advice from the country they are dealing with.
Just a few differences between French and English law include the system (Roman/Napoleonic law as opposed to common law), the courts (e.g. the French Court of Cassation, which has similarities but is not equivalent to the High Court), legislation (French Decrees and Laws as opposed to British Acts), types of lawyer, and, above all, terminology. The arcane formulas often encountered in legalese date back centuries (in both cultures) and in many instances are not directly translatable.
Clients may also have to consider the type of English they want for their translation. Within the UK, there are differences between English and Scottish law that may require translator expertise in one system or the other. Translations for the United States should really be undertaken by a US translator, not just because of differences in spelling and vocabulary, but because of differences in law there.
In short, a translation will aim to be as informative as possible but precise accuracy may be thwarted by the major differences referred to above. At the end of the day, the client should get a clear picture from the translation of what the document is about, but should refer to a legal adviser of the country of origin if they need precise information and instructions on the exact terms used in the original.