The translator swears it, not you
The whole document turns on one oath, and it is not yours. A certified translation of a Translator's Affidavit in Cardiff is sworn by the translator, who states in the first person that the finished text holds true to the source. The page gives their credential, both languages and the date. It is bound to the translation so nobody can swap the pages later.
A bilingual source page raises the bar
Cardiff throws in a wrinkle that most cities do not. Civil records registered in Wales are frequently issued in Welsh and English side by side, so the translator is reading two languages before writing a word of the third. The declaration must be honest about that: it names what was worked from, and it covers the whole page rather than the English half.
The witness, and what the seal covers
The oath is then taken in front of a solicitor or a notary public. They take proof of who the translator is, watch the signature go down, and add a seal with the date. That is the entire scope of it. The seal fixes identity and the act of signing, while accuracy remains the translator's word, and the two together make an official translation of a Translator's Affidavit in Cardiff.
Order the copy, then the same national rule applies
Where the sworn text sits on a civil record, get a fresh certified copy before the translator starts. Cardiff Register Office at City Hall, Cathays Park, Cardiff CF10 3ND issues copies of births, marriages and deaths registered in the city, so your own certificate never has to move. After that the rule is the same countrywide: a certified translation of a Translator's Affidavit in United Kingdom practice is a witnessed accuracy statement from a qualified translator.