This article is from the FAQ, by with numerous contributions by others.
[Addition suggested by Siegfried Rambaum]
Immigration related documents may be translated by any person who is
competent in both the foreign language as well as English. It is best to
ask a person not related to you to translate. Each translation must be
accompanied by substantially the following words:
"I, [Name of Translator], do hereby certify that I am proficient in both
English and [Foreign Language], and accordingly I am competent to perform
translations, and that I have translated the foregoing/attached document
accurately and to the best of my knowledge and ability.