What you already own
Begin by setting aside the record you hold. A certified translation in Southampton for Democratic Republic of the Congo has to be in French, and none of the work needs the paper in your drawer to go anywhere. For a civil record, Southampton Register Office at 6a Bugle Street, Southampton SO14 2AJ issues a fresh certified copy that does the job in its place.
What gets made
Two things come into being. The first is the French version of your text, sheet for sheet. The second is the certificate the sworn translator signs, carrying a name, a date and the oath behind it. Neither existed before you asked, and both are dated, which is why the work is quoted at about £25 / page rather than sold off a shelf.
What travels and what stays
The certified copy and the French set go out together as one file. What stays behind is your own record and, if you plan for it, a spare set kept clean at home. Standard work takes 2–4 business days and express takes 24–48 hours, so folding a second set into the same order costs you the waiting time only once.
The signature that goes with it
Sworn work is required on this route, and the credential is named: traducteur assermenté, a translator appointed to certify on oath. The oath is written on the certificate, along with the translator's name and the date the pages were done. For as long as the file exists, that certificate has to stay physically with the pages it describes.