I am going back before Brexit so may no longer apply, but when we went for our CdS it was January 2nd. I was putting together our final paperwork over Christmas when I realised I should have got official French translations of our marriage and birth certificates. It was impossible to get a translator at that late stage, so I prepared my own French translation using google and reverso which is good for technical / official language. I made my work look as official as possible, signed it and dated it. My translations were accepted without a murmur. But I had no choice and got away with it. If you have time / funds then probably a certified translator (I think it’s a traducteur assermenté - see Pages Jaunes).
I’ve uploaded a couple of pdf files showing what I produced. In fact, if you think about it, the translation skills required to translate birth and marriage certificates are minimal because the documents are mainly names and addresses and dates. Hope this helps. It might mean you can prepare them yourself and then wave them under the nose of an official translator and just get their stamp on them.
Marriage certificate full translation example.pdf (136.6 KB)
Sue birth certificate full translation example.pdf (133.8 KB)