firefox-locale-mn software package provides Transitional package for unavailable language, you can install in your Ubuntu 16.04 LTS (Xenial Xerus) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install firefox-locale-mn
firefox-locale-mn is installed in your system.
Make ensure the firefox-locale-mn package were installed using the commands given below,
$ sudo dpkg-query -l | grep firefox-locale-mn *
You will get with firefox-locale-mn package name, version, architecture and description in a table
| Distro | Ubuntu 16 04 lts xenial xerus |
| Section | Web Software |
| Package | firefox-locale-mn |
| Version | 50.0+build2-0ubuntu0.16.04.2 [ |
| Description | Transitional package for unavailable language |
| Sub Section | web |