The title is a reference to Agent 355, which was the codename of an unidentified female spy who fought for the Patriots during the American Revolution.
Marie (Diane Kruger) is seen speaking three languages at different points in the film. They are English, French and German. The actress is fluent in all three languages.
Diane Kruger said Jessica Chastain made sure all of the other actresses in the lead cast ensemble were all paid the same amount.
Jessica Chastain proposed the idea for a female-led spy film in the same spirit as the Mission: Impossible and James Bond series to her X-Men: Dark Phoenix (2019) director Simon Kinberg while in production on that film.
In the auction scene where Nick (Sebastian Stan) is instructing the guard to keep Mace (Jessica Chastain) in the room, he's speaking Romanian. Stan is Romanian-American and can speak fluent Romanian.