firefox-esr-l10n-en-za software package provides English (South Africa) language package for Firefox ESR, 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 firefox-esr-l10n-en-za
firefox-esr-l10n-en-za is installed in your system.
Make ensure the firefox-esr-l10n-en-za package were installed using the commands given below,
$ sudo dpkg-query -l | grep firefox-esr-l10n-en-za *
You will get with firefox-esr-l10n-en-za package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Language packs |
| Package | firefox-esr-l10n-en-za |
| Version | 45.4.0esr-2 |
| Description | English (South Africa) language package for Firefox ESR |
| Sub Section | Localization |