Run time detection: Does the terminal emulator support unicode?

You can test the terminal by setting the cursor position to column 1 and outputing a multibyte unicode character. If the cursor moves by more than 1 position then the terminal does not support unicode.

On in this case we emit a 3 byte sequence which is a zero width space, so if the cursor moves at all, the terminal cannot process unicode

IFS=$';\x1B[' read -p $'\r\xE2\x80\x8B\x1B[6n\r   \r' -d R -rst 1 _ _ _ X _ </dev/tty 2>/dev/tty && test "$X" = 1

Here we output \r to get to position 1 and then emit a 3 byte sequence which is a zero width space, and then emit ESC [ 6n which asks the cursor position, followed by \r \r to overwrite any junk that will have appeared if the terminal handled each byte as a separate character.

Then we read the cusor position with a 1 second timeout and check whether the X position is position 1 which it will be if the terminal can process unicode.

A better function is:

is-tty-unicode() {
  local X

  test -c /dev/tty &&
  if test -t 0
  then IFS=$';\x1B[' read -p $'\r\xE2\x80\x8B\x1B[6n\r   \r' -d R -rst 1 _ _ _ X _ 2>&1
  fi <>/dev/tty && test "$X" = 1
}