Five issuing authorities cover the bulk of UK clearance translation work.
India: the Police Clearance Certificate (PCC) is issued by passport offices or local police authorities. The standard PSK-issued PCC is bilingual Hindi-English, with the English section carrying the binding text. If the PCC is the standard format, no translation is needed. If it's a state-police-issued PCC in Hindi or a state language, the certificate needs full translation.
Pakistan: the Character Certificate is issued by district police authorities and is usually Urdu only. Some district authorities add a brief English summary block, but the binding text is the Urdu. Full certified translation is needed.
China: the No Criminal Record certificate is issued by the local Public Security Bureau (PSB), is Chinese only, and requires Chinese notarisation before it leaves the country. The notarisation booklet adds 4–8 pages of formal Chinese legal text around the certificate itself. UKVI wants the whole booklet translated, not just the certificate page.
Nigeria: the Police Report from the Nigerian Police Force is issued in English on official letterhead. No translation is usually needed. Edge case: a state-command-issued report with a local-language stamp.
Philippines: the NBI Clearance is issued in English. No translation needed.
Elsewhere: most European clearances (Spanish Certificado de Antecedentes Penales, Italian Certificato del Casellario, French Bulletin n°3) are in the issuing country's language and need certified translation.