Mongolian is the language Mongolia works in
Papers used in Mongolia are read in Mongolian, so a certified translation into Mongolian in Newcastle upon Tyne is what makes an English record usable there. That applies to the document as a whole, whichever version of it you choose to send: an old sheet you have always had, or a copy issued last week.
One reproduces, the other renders
A certified translation for Mongolia is a new document. It did not exist before, it carries a translator's name, and it stands or falls on how faithfully it renders the source. A certified copy is not new at all — it is your entry printed out again by the office that holds it. Mixing the two up is easy, and it costs a week when the wrong one turns up.
Where each one comes from
Certified copies of births, marriages and deaths come from Newcastle Register Office, at Civic Centre, Barras Bridge, Newcastle upon Tyne NE1 8QH. That office does not translate, and it does not certify translations, so none of the Mongolian work is available across its counter. The pages are written separately, from an image of whichever sheet you have settled on for the file.
What the Mongolian carries
It carries a statement of accuracy, signed and dated by an ITI/CIOL-qualified certified translator and naming the record it was made from. An official Mongolian translation in Newcastle upon Tyne rests on that single sheet, so keep the sheet with the pages it belongs to. Split apart, neither half says much: the pages lose their author and the statement loses its subject.
Cost per page and the order of things
The Mongolian costs £25 for each page as issued, so the number of sides decides the bill and nothing else does. Standard work takes 2–3 business days once a clear scan lands, and express takes 24 hours. Ordering a fresh copy of a civil record carries its own wait, so start that step early rather than after the pages are already written.