Don't confuse this with Displays Escape Sequences. If what you type or see on the screen is not what's expected but looks like a foreign alphabet, math symbols, line-drawing character, etc. then it could be that the many of bytes that are sent to your terminal have the high order bit set (when it shouldn't be). You are looking at the character set (or part of a character set) which has the high order bit set. This may happen if you have the baud rate wrong or parity set wrong (per stty). If you have parity set per stty but inside the terminal it's 8-bit no-parity, then the high order bit (= parity bit) will often be erroneously set. Try stty -F /dev/ttyS? from another terminal to check if the baud rate and parity are correct.
Perhaps the wrong character set (font) has been installed. An erroneous escape sequence sent to the terminal could have switched character sets. If you are using the mapchan program to change the keyboard mapping, it could be incorrect.