"legata" did exist, and the dictionary says it was a female ambassador, rather than a female military officier (which I doubt did exist at this time, even among amazons)
Legata is the feminization of legatus.
Legare means "to send, envoy" (here: someone)
Gave legate in English, via the French légat.
Ambassador is from French Ambassadeur, this one borrowed also from the Italian "Ambasciatore" from Ambasciata (« embassy »), from Latin ambactus (servant, slave).