Update main.it.yml

Here proposed a complete transaltion in Italian Language.

I'm not sure about the perfect transaltion of some words (in relation to salorhospitality application context); examples:
1. Item, Items => Articolo, Articoli / Item, Items (these English terms are commonly used in Italian technical "dialects")

2. Invoice, Invoices => Scontrino, Scontrini / Fattura, Fatture / Ticket, Tickets / Ricevuta, Ricevute

notes:
to indicate a paper list of items (to be billed) in a general sense, all valid translation of 
term "Invoice" 
could be: Scontrino, Ricevuta, Conto, Ticket. 

Receipt => Ricevuta
Fiscal Invoice => Ricevuta Fiscale

I transalted 
Invoice => Scontrino (Scontrino literally mean "ticket") 

3.some terms have no clear transaltion in Italian, by example
to indicate the handset used by waiter to tale orders:
  
waiterpad => waiterpad / handset 
workstation => workstation