Sorry, mouse.   Richard is correct. People who say "the hoi polloi" show not only their ignorance of another language, but the fact that they don't care they are ignorant.

That sort of attitude, especially by Americans, is part of the reason why Americans abroad are so popular.   Hence "ugly American."   

And it makes Americans easy marks for comedy by those who do know more than just English.

It doesn't take much to do a bit of research on a word--or just don't use it.

Shades of Mrs. Malaprop.