The load-bearing line on GOV.UK is short: bring "a translation of any documents that are not in English". The guidance does not say who may translate, does not use the word certified, and does not mention sworn translators. It also does not say what happens at the appointment if a translation is missing or the registrar doubts it. The pages we checked never ask for your decree to be apostilled or legalised either.
In practice, many couples choose a certified translation anyway, because it carries the translator's signed confirmation and removes doubt on the day. That is a choice, not a stated requirement. GOV.UK's own general guidance on certifying documents adds: "Check with the organisation that needs the certified copy - they may have specific rules for who can certify a document." Your register office is that organisation, so if you want certainty, ask them before your appointment.
One more visa warning from the official guidance: you cannot give notice, get married or form a civil partnership on a Standard Visitor visa. If you give notice without the right visa, the Home Office will be told, may ask questions about you and your relationship, which can mean waiting up to 70 days, and might decide not to approve your notice. This page covers England and Wales.