A "legalised" passrport is legal when it is a genuine one issued by the competent authorities of the holder's country.

How a Chinese authority can "legalise" a legal passport from anozher country is beyond me.

But perhaps it is only my intelligence that is too limited .....LOL

Very well reasoned but that's the word they used. Even if they mean translate it doesn't make much sense. Translate what-place and date of birth and expiry; all of which can easily be deduced?