What’s the difference? An affidavit is a notarized, sworn statement from the witness or party to a case giving his or her account of the facts. A declaration is the same statement, unsworn (not notarized). [source]
When translators in the United States certify their translations for a client, very often a declaration can be used instead of an affidavit (though many times clients opt for the extra level of formality and have a notary get involved—just in case).
Section 2015.5 here sets out the language you should use to make sure your declaration can hold its own:
I certify (or declare) under penalty of perjury under the laws of
the State of <insert state name> that the foregoing is true and correct:
If you change things up to make this read better for a declaration about your translation, be sure you have the elements “under penalty of perjury,” “laws of <place>,” and “true and correct.” You should also, at the beginning of the statement, give information about who you are and how you are qualified to make such a statement.
As always, for legal advice for specific situations, consult a qualified legal professional (in this case, a notary should be able to help).
What are the laws about affidavits versus declarations in your country or state? Do you make declarations about your translations often? What language do you use?