DVLA requires a certified translation when you're exchanging a foreign driving licence for a UK licence and the original isn't in English, or when the police request to verify your licence during the 12-month transition window after becoming a UK resident. The translation must include the issuing authority, all dates, categories, and any restrictions.
The exchange route depends on the country. Designated countries (Australia, Canada, Singapore, South Korea, and a few others) get a straight exchange without a UK driving test. Most other countries need a UK provisional licence and the standard driving test, but DVLA still wants the original licence translated as part of the application file.
The translation captures the licence categories (B for car, BE for car with trailer, A for motorbike, C for HGV, D for bus, etc.), restriction codes (lenses required, automatic-only, etc.), and the issue and expiry dates. Photograph references and signatures are noted but not transliterated.