Bytes run from -128 (0x80) to +127 (0x7f). The characters used in Western European languages other than English are in the range 0x80-0xff, so they are regarded by two's complement as negative numbers. You can find the numbers in
Unicode (1) and
(2). I note some of those characters in no (2) are control characters.
Not sure what you are supposed to do next, but it has something to do with casting to a char, or casting to a char and doing a bitwise AND (&) with 0xff.