aegisub-l10n software package provides aegisub language packages, 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 aegisub-l10n
aegisub-l10n is installed in your system.
Make ensure the aegisub-l10n package were installed using the commands given below,
$ sudo dpkg-query -l | grep aegisub-l10n *
You will get with aegisub-l10n package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Language packs |
| Package | aegisub-l10n |
| Version | 3.2.2+dfsg-3 |
| Description | aegisub language packages |
| Sub Section | Localization |