Force different language as primary

[Lady R]

Hello, I'm new there and I'm Italian, so sorry for my bad english ^.^"

I've a website with two languages: English (primary) and Italian.
I've also a Virtuemart store, available only for Italian users.

I would like to set the primary language of Virtuemart as Italian, but keep the Joomla's primary language as English.

I think I need to edit some file, when Virtuemart reads the default language, and hack it for set Italian instead of English, but I don't know where find it.

May someone help me?
Thank you