Not the same thing:
girlfriend = current partner (i.e. until the next one show up)
wife = long time or stable partner
concubine = mostly a legal term only for people who have signed that specific partnership contract.
so that's their definition in French?
To me though, gf = current partner, wife = current partner but you just want it in writing, concubine = an ancient term, modern term is mistress, google Tiger Wood.