I'm no expert on this, but the question intrigued me so I looked around a little. It would appear that it depends a lot on the purpose of the translation and to whom it is going to be presented. For example, translation of a letter for a private individual would require no certification at all unless the individual required it as part of your agreement with him. Translation of official documents is a very different situation. There are a number of organizations that appear to provide training and could define what certification is needed in different situations.

