Localisation of Debian
After Debian is installed on the BeagleBone, the localization must be adjusted so that the correct fonts are available.
By default, the location is set to "en_US", but this font can not display German characters and some special characters.
So the correct fonts must be installed.
So in Debian type the command
dpkg-reconfigure locales
A menu will appear where you can easily select to install fonts.