I agree with SneakyPete, and I believe the I-129f INSTRUCTIONS PDF file confirms/clarifies this, as well.
We had a hell of a time getting our translation company to put the certification wording that Pete showed on the translated documents. Translation company insisted that their company stamp would be enough. Eventually, we had to tell them to add the statement or stop getting paid.
At the end of the day, you can even have a bilingual friend or family member do the translation, as long as they put the certification statement on the document.