[Campion]

I think “let’s go Francis” would be imperative, “Ite Francisce!”, or something like it. “Ite Francisce” is “Go, Francis!”. I’m not sure what to do with “Let’s”; it’s idiomatic.

[Dr. Sivana]

The "Let's " is why I was drawn to the subjunctive. "Allow us to go", "That we may go". "Ite" works for "Go", but that not only drops the "Let's" but it is singular, so the "us" is also dropped.

Oh well, as the brilliant polyglot Fr. Amar pointed out (no doubt quoting another) "every translator is a traitor".