The man su was new to me.

One of the first things to know with any UNIX variant such as Linux is that man pages are the first place to go when you have problems. Whenever you see a recommendation to use a command you are not familiar with you should always start by reading the man page for it to see what it will do. Man itself has a man page (enter man man to see it).

Also try:

echo $PATH
su
echo $PATH
exit # or press control-d
su -
echo $PATH

Trying things out is a good way to learn how Linux works under the hood.

Anyway, I'm glad you got things fixed.

# dpkg-reconfigure locales

This is useless:

# dpkg-reconfigure

And try comparing output from echo $PATH after su and su - (also output from env and set, preferably piped into less).

Using the "Search" function of these forums would also have answered your query. We have had several threads here about the new su behaviour.

su -
apt reinstall locales
locale-gen

The dpkg-reconfigure command does not work after that.
Terminal error message =

root: dpkg-reconfigure
/usr/sbin/dpkg-reconfigure: bitte geben Sie ein Paket an, das Sie neu konfigurieren wollen

With only su the locale-gen command does not work and with su - the command works.
What is actually the difference between su and su - in terminal?

the following command doesn't fix the problem either.
echo "export LC_ALL= " >> ~/.bashrc

Remove that added line from ~/.bashrc

Looks like your locales are messed up so try

# apt reinstall locales

Then edit /etc/locale.gen to un-comment de_DE.UTF-8 & en_US.UTF-8, save the file and run

# locale-gen

Does dpkg-reconfigure work after that?

#01 normaluser: su -
#02 Passwort:
#03 -bash: Warnung: setlocale: LC_ALL: Kann die Regionseinstellungen nicht ändern (en_US.UTF-8): Datei oder Verzeichnis nicht gefunden
#04 root: dpkg-reconfigure locales
#05 Generating locales (this might take a while)...
#06   de_DE.UTF-8... done
#07   en_US.UTF-8...[error] cannot open locale definition file `en_GB': No such file or directory
#08  done
#09 Generation complete.
#10 root: locale -a
#11 C
#12 C.UTF-8
#13 de_DE.utf8
#14 POSIX

Doesn't change anything either. I can execute the dpkg-reconfigure locales command but I still get the error message #03 & #07 .

Use

su -

Do not use plain su, that will not work. You need to add the dash.

I changed something earlier and I don't know how to fix the first error message (#03) in the terminal and the following command doesn't fix the problem either.
echo "export LC_ALL= " >> ~/.bashrc
I get the message number 03 whenever I become root. What can I do if I get error number 09?

Regards.