Sworn is a status, held in advance
A certified translation for Chad has to be sworn, and the person who swears it is a traducteur assermenté — a translator formally appointed to give sworn translations. The appointment exists before your document does. You are not commissioning an oath about your birth entry; you are commissioning work from someone whose signature already carries one.
Who can take the job
Because the credential is held rather than claimed, it decides the routing of the work. A certified translation into French in London on this corridor goes to a signatory who holds the appointment, and not to whichever translator happens to be free. That is a real constraint on scheduling, and it is the honest reason the standard window is quoted as a range.
It also means the job is not split. One appointed person reads the sheet, writes the French, checks it against the image and signs the result, so a single name stands behind every line. Passing half a document to a second pair of hands would leave two people vouching for one signed set, which the sworn form does not allow.
French is what this corridor asks for
Papers put to official use in Chad have to be in French, so the target language is fixed before anything else is decided. An official French translation in London is therefore not a preference you weigh up; it is the starting condition, and the sworn credential attaches to that language rather than to any particular kind of document.
Rate, window and what you actually decide
Work is about £25 a page, with 2–4 business days as standard and 24–48 hours on express. Those two numbers are the whole of your decision, because the credential, the language and the form of the signed statement are all set by the corridor. A certified translation in London for Chad leaves you the deadline to choose and little else.
Where the paper comes from
Nothing in this needs your original in an envelope. Westminster Register Office at 317 Harrow Road, London W9 3RJ issues certified copies of birth, marriage and death entries, and a certified copy is imaged and translated exactly as the first one would be. That office works by post and by appointment, and it neither translates nor swears anything itself.