Yes documents not in English do indeed need to be translated into English ... to assist the ECO in the British Mission to make their decision. After all that ECO is British and is not necessarily fluent in the language of the country where they happen to be stationed.
Those translations could also be helpful in the UK, so don't neglect to bring them with you. For example, a translation of a birth certificate will be helpful in getting a National Insurance number or driving licence, etc etc..
General rule in the UK .... use the original document
and the translation into English as if they together formed one document. That is, don't just supply the translation ... supply the original and the translation together.
... by a sworn translator?
Check with the British Mission concerned whose translation they will accept. For example in Bangkok where my wife applied the British Embassy will accept a translation done by any of the translation bureaux in Bangkok or elsewhere, and certified by that bureau.
