Setting the locale is documented in the Debian install guide - there's an appendix which provides some hints on installing directly with debootstrap and configuring the system yourself.
To configure your locale settings to use a language other than English, install the locales support package and configure it. Currently the use of UTF-8 locales is recommended.
# aptitude install locales
# dpkg-reconfigure locales
The appendix as a whole has a disclaimer that it is not comprehensive, but it is official documentation and this specific method is perfectly correct. There are other alternatives which may be preferred for scripting - this method prompts the user to choose which locale(s).
There is a second issue which the appendix also mentions in passing. I am not sure if it affects your specific character issue, but it can cause issues with similar sophisticated output. You need to make sure that TERM is set correctly. Run
echo $TERM outside the container. Inside the container, run e.g.
export TERM=xterm-256color to set the terminal type for this session.
I don't think
machinectl login handles this for you either, which is sad given how it talks to systemd inside the container.
If you run an SSH server inside the container, then just use that, SSH will forward TERM correctly and you don't have to do anything.