The language Japan asks for
Japan works in Japanese, so a UK record filed there needs a Japanese version. Written Japanese mixes kanji with two kana scripts, and a name from a Latin alphabet has to be set out in a form a Japanese clerk can read and match. A certified Japanese translator does that mapping, then signs for it. A guessed name or a shifted date is what sends a file back.
The certificate Japan takes
The form Japan takes is a certified translation with a certificate of accuracy. The translator signs a short statement: this is the source, this is the target language, and the second text says what the first one says. An official Japanese translation in Reading is only finished with that page on top. Without it you hold a good translation and no proof of it.
Cost and the clock
You pay by the page of the source, at about £25 per page. Standard turnaround is 3-5 business days, counted from the hour the scan lands with us. Express work comes back in 48 hours and suits a visa slot or a court date that will not move. A birth certificate is usually one page; a company file runs longer, so the cost tracks the paper.
Using the file in Japan
File the Japanese text and the English source together, with the certificate on top. Japanese desks are exact about matching, so a set that has been split or reordered reads as incomplete. Where the record is a civil one, order a fresh certified copy first and leave your own paper at home. Then nothing you cannot replace is riding on a courier van.