Do martial artist have to warn people if they get in a fight?

No.

Why? It is none of their business what you do in your free time. The law basically say that you have to live with the consequences of your actions. Someone attacking you is willing to take the risk that they will lose. They probably can't imagine it happening, but that is the risk they decided to take.

So, are you supposed to stop and show them your certificates and trophies? What amount of training would you have to have in order to be required to warn them. Is one class enough? Does a yellow belt have to provide the same warnings as a 5th degree black belt? Does the style matter?

At the same time, you have to be willing to live with the consequences of what you do, which might mean their death or permanent injury.

And being a martial artist, you would never attack them, defense only.