So let's break her name down and figure out out, shall we?
One other thing to note is that in the materials artbook her name is simply 'Sailor Metal Papillon.' How the 'heavy' got stuck in her name for the manga we will never know, but somewhere along the line Naoko decided to change it.
'Heavy metal' is a loose term that refers to highly dense metals such as the transition. Many of the well-known poisonous metals such as arsenic and lead are considered heavy metals, as well as many of the transition metals. As Sailor Heavy Metal Papillon is the most powerful Animamate it isn't surprising that instead of a specific metal - such as aluminum or iron - she is giving a more 'all-encompassing' name befitting her position.
Papillon is French for butterfly (hence the butterfly theme). For some reason, this part of her name is commonly misspelled (it is not Papillion, Papilion, Papilon, etc). As for the correct pronunciation, you say it with a "Y" sound instead of an "L" sound, so it's more like pah-pee-yawn.
Just a little note, I know you could write her name as Sailorheavymetalpapillon, but I personally choose to put spaces between the words because it makes it less of a mouthful (eyeful?).
The Sailormoon manga isn't only available in Japanese, so there are countless languages that Sailor Heavy Metal Papillon has been translated into! Except she mostly seems to stay the same... Cantonese clearly is the winner here!
Cantonese: Sailor Heve Papillon
French: Sailor Heavy Metal Papillon
English: Sailor Heavy Metal Papillon
Spanish: Sailor Heavy Metal Papillon
German: Sailor Heavy Metal Papillon
If you have a non-japanese version of the manga I'd love to hear from you as to what she's called!