This has to do with your system not have a UTF-8 locale by default.
Since you are running Debian this is an easy fix. Please run
sudo dpkg-reconfigure locales
which will bring up a menu to allow you to select a locale. For me I would use en_CA.UTF-8 for English/Canada. Make sure your locale has a UTF-8 ending. Now press OK, and on the next screen make sure this locale is the default one. Then media-monitor should run.