If you are applying for leave to remain or indefinite leave to remain, the Immigration Rules add a fifth element: "certification by a qualified translator and details of the translator or translation company's credentials". The guidance never defines "qualified translator" and names no register, so the practical answer is a certification that states the credentials in full.
The cost of getting this wrong is spelled out. If no translation is supplied, the caseworker requests one. If you still do not provide one, or the translation cannot be verified, "the document will not be accepted" and the application is processed as if you had not provided the document at all. A missing specified document means the requirement it evidences is not met.