This isn't a question we can answer simply: it may not even be possible.
If you mean that you have a terminal as in a VDT (a dumb terminal device such as a VT220, or similar) connected to your PC via a serial port (RS232 or similar) then it is going to depend on exactly what the terminal itself can display, and exactly how you tell it to do that. I know of no terminal that uses Unicode (heck, I didn't know they still made VDTs!) - they all used ASCII in my day, which may present a solution to your problem.
Instead of sending your data to the terminal as a string, send it as a byte array instead, and use ASCII encoding instead of Unicode:
string s = "Hello!\r\n'être et avoir' is a French documentary, made in 2002";
byte[] bytes = System.Text.Encoding.ASCII.GetBytes(s);
mySerialPort.Write(bytes, 0, bytes.Length);
I don't guarantee it will work for all characters you need, but it stands a better chance!