I wouldn't say it was 'a translation error', just a not-very-sensitive transliteration. There is, as a matter of fact, a cheap(er) sort of silver called EPNS (Electro-Plated Nickel Silver), often referred to as 'plate'.
(Incidentally, I wonder if de la Plata has anything to do with tha naming of Argentina - which itself means 'silver'.)
ps - I don't think 'Plate' (as in plate silver -> cheap) was a deliberate insult. As I said, it's a transliteration rather than a translation.