You do not need to have training in the law to be a legal translator. You do need to understand "legalese". There is a difference.
If you’re going to translate legal documents, you need to understand your source text. You also need to express it properly in your target language. Legal terms often have meanings in a legal context that differ from common meanings. A specific example:
A translator’s English source text, from a long contract, which he’s been asked to translate into Spanish:
Contractor shall give lectures on the dates specified in Exhibit 8 hereto.
Looking up "exhibit" in a well-known English to Spanish dictionary, he finds:
2. (legal) prueba instrumental
Okay, he says, this is a legal document. So he writes:
El Contratista dará clases en las fechas que se indican en la Prueba Instrumental 8 al presente.
Guess what, this isn’t what the English says. "Exhibit" in this context is not evidence of anything, so it isn’t "prueba instrumental". Lawyers often write contracts and other documents using the word "exhibit" to mean "attachment" or "enclosure". This translator is either unaware of what "exhibit" means in this case or is unaware of the meaning of "prueba instrumental" (or maybe both). While "prueba instrumental" would be fine for an exhibit introduced into evidence in a courtroom, a more appropriate translation here would be "Anexo".
Any treaty entered into between the United States and a non-English-speaking country must, by law, have the English text checked by State Department personnel to determine whether it matches the other-language version. As such, there is an official English text and an official other-language text. If you get quotes from one version for translation into the other language, you need to find the other version. If you can't find it, you need to ask your client for it. As a last resort, note that your translation is not the official text.