After installing and deinstallig a language, it is left behind in the LANGUAGE system variable

After removing an installed language that was previously selected, it's still defined in the system. This leads to many UI elements and software applications picking the previously installed language if there is no translation available for the currently selected language.

Steps to reproduce:

1. Open GNOME Settings
2. Install the Deutsch language pack
3. Select it as your system language & locale
4. Apply settings
5. Pick a different language and uninstall the Deutsch language pack
6. Reboot

This is the result with `gd` as my usual language/locale:

$ locale
LANG=gd_GB.UTF-8
LANGUAGE=gd:de_DE:en_GB:en
LC_CTYPE="gd_GB.UTF-8"
LC_NUMERIC=gd_GB.UTF-8
LC_TIME=gd_GB.UTF-8
LC_COLLATE="gd_GB.UTF-8"
LC_MONETARY=gd_GB.UTF-8
LC_MESSAGES="gd_GB.UTF-8"
LC_PAPER=gd_GB.UTF-8
LC_NAME=gd_GB.UTF-8
LC_ADDRESS=gd_GB.UTF-8
LC_TELEPHONE=gd_GB.UTF-8
LC_MEASUREMENT=gd_GB.UTF-8
LC_IDENTIFICATION=gd_GB.UTF-8
LC_ALL=

Notice the `LANGUAGE=gd:de_DE:en_GB:en` bit that should read `LANGUAGE=gd:en_GB:en` instead.

I also noticed that after switching the language back to `gd` while keeping the `de` locale, part of the UI was still in `de`, including the "Manage Installed Languages" button, while other parts were in `gd`. That's probably the same bug and a symptom of the incomplete translation coverage for `gd`.

I managed to manually sort of fix it by running sudo update-locale LANGUAGE=gd:en_GB:en as explained in https://askubuntu.com/questions/1083868/how-to-set-language-in-locale-permanently/1084062#1084062. However, it will regress every time I "Manage Installed Languages" -> "Apply System-wide". See attached screenshot for the result.

Originally reported at https://gitlab.gnome.org/GNOME/gnome-control-center/issues/236

Ubuntu version: 18.04