Most of the docker images use base debian 9 image which only contains english locale. To add other locale definitions do the following after starting the container image:
apt-get -q update apt-get -yq install locales sed -i 's/# de_DE.UTF-8 UTF-8/de_DE.UTF-8 UTF-8/g' /etc/locale.gen sed -i 's/# en_US.UTF-8 UTF-8/en_US.UTF-8 UTF-8/g' /etc/locale.gen locale-gen update-locale 'export LC_ALL="de_DE.UTF-8"' . /etc/default/locale
This enables and installs English usa and German locale and defines German as default