installation-locale software package provides UTF-8 C locale, you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install installation-locale
installation-locale is installed in your system.
Make ensure the installation-locale package were installed using the commands given below,
$ sudo dpkg-query -l | grep installation-locale *
You will get with installation-locale package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | debian-installer udeb packages |
| Package | installation-locale |
| Version | 1.6 |
| Description | UTF-8 C locale |
| Sub Section | Debian-installer |