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
A menu will appear where you can easily select to install fonts.
Now mark the corresponding entry "*.utf8"-entry of your country.
Important: Do NOT deselect the entry "en_US.utf8". This might lead to an unstable system.
Accept the setting with OK. It appears another menu where all the installed fonts are listed. Select now the default character set of your system.
Here now simply set corresponding *.utf8-entry and confirm with OK. After that the new character sets are created, which can take a little while.
And that's it.