After upgrade to v6.0.0, en_GB locale no longer listed

Steps to reproduce:

1. Upgraded from 5.4 to 6.0
2. Dashboard was complaining that en_US was not installed, and it wasn't because my RPi is set to only en_GB
3. Installed en-US locale and dashboard was fine
4. Checked in general settings and user profile, Language is set to "System default" but no option to choose en_GB is listed - other languages are including the now installed en_US

Result:
See screenshots

[email protected]:~ $ locale
LANG=en_GB.UTF-8
LANGUAGE=
LC_CTYPE="en_GB.UTF-8"
LC_NUMERIC="en_GB.UTF-8"
LC_TIME="en_GB.UTF-8"
LC_COLLATE="en_GB.UTF-8"
LC_MONETARY="en_GB.UTF-8"
LC_MESSAGES="en_GB.UTF-8"
LC_PAPER="en_GB.UTF-8"
LC_NAME="en_GB.UTF-8"
LC_ADDRESS="en_GB.UTF-8"
LC_TELEPHONE="en_GB.UTF-8"
LC_MEASUREMENT="en_GB.UTF-8"
LC_IDENTIFICATION="en_GB.UTF-8"
LC_ALL=
[email protected]:~ $ locale -a
C
C.UTF-8
en_GB.utf8
en_US.utf8
POSIX

Expected: