Judge in Dream: I accuse you... of a wasted life.
Papillon: Guilty... guilty... guilty.
Warden Barrot: Put all hope out of your mind. And masturbate as little as possible, it drains the strength.
Papillon: We're something, aren't we? The only animals that shove things up their ass for survival.
Dega: Blame is for God and small children.
Maturette: I'll do what you ask, on one condition: You keep your money and you take me with you.
Papillon: No.
Maturette: I'm the only one who can get you out of here.
Papillon: You're.
Maturette: I know. I'm a queer, a fairy, a poof, huh? But there's one thing you forgot. You may have been framed, as you say you were... but I wasn't. Between the two of us... I'm the one who's killed a man, not you.
Papillon: Okay.
Dega: If I stay - here in this place -I will die.
Toussaint: How did you know I have dry leprosy, that it isn't contagious?
Papillon: I didn't.
Maturette: We all have sensitive spots.
Camp Commandant: Make the best of what we offer you, and you will suffer less than you deserve.
Toussaint: We do a lot of smuggling here. We raid the mainland. We steal boats. When an outsider comes in we generally kill him, as a security measure.
Papillon: That makes sense.
Toussaint: Well... a man of Christian understanding.
Papillon: Hey you bastards, I'm still here.
Toussaint: If you're going to catch leprosy, it's better to catch it from money than from people.
Warden Barrot: Papillon made it to freedom. He lived out the rest of his natural born life. The Penal Colony did not.
Answer: It does appear to be unintentional (or at least, unscripted), but they continue because when you're shooting a movie, you don't stop until you hear "cut", and especially at that time animal welfare wasn't necessarily a priority. I'm guessing no-one was concerned about the chicken, and so didn't feel the need to do anything about it. It's possible the film was made without an animal welfare monitor on set. As to why it's in the movie, the whole "marching to the prison" sequence was probably handled by the assistant director (as shots like this, not involving the principals or any substantive dialogue, often are) and they may have only done the one take. Who knows, they may have thought the injured chicken added realism to the scene.