Which documents and what type of translataion depend on the specific consulate.
Generally speaking, the only docs requiring translation are your own (plus spouse and children, if applying with you) birth and marriage certificates. These can usually be done by anyone, including yourself. If at all possible, they should be done in a manner to look like the original, although this is not strictly a necessity - I was told by New York, for example, that I could simply list the items numerically (most long form documents seem to have the data boxes numbered...). Some consulates insist that you use a translator chosen from a list they provide. Many consulates state that translations should be performed "professionally" but this seems to mean "in a professional manner" as opposed to "by a professional translator" - you should clarify this with your specific consulate.
If you have been through one (or more!) divorces, the divorce decree must be translated as well and always, from what I have seen, by a professional who is competent to translate legal documents.
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