They are different jobs. An interpreter helps you understand and speak at the hearing, and GOV.UK says family courts provide one if your case involves children, domestic violence or forced marriage. Written evidence in another language still needs translating before it goes into the bundle.
An interpreter works with spoken words in the courtroom. Under GOV.UK's guidance, you'll be given one in family cases involving children, domestic violence or forced marriage, and in other family cases only if you can't afford one, don't qualify for legal aid and have no friend or relative the judge will accept. If you want a friend or relative to interpret, you must ask the judge for permission before the hearing.
A translator works with the written documents, usually weeks before the hearing, so the judge and the other side can read them in advance. Booking an interpreter doesn't get your documents translated, and translating your documents doesn't mean you won't need an interpreter. Many people need both, and it's worth sorting the written side early, because bundles have deadlines.