ubiquity removes packages for non-en_US english locales

To reproduce:

Proceed through a normal installation from an Ubuntu ISO (here I was trying today's cosmic daily 20181012). Pick an English (UK) keyboard layout and click on the UK when you see the location map.

Reboot into the installed system, open a terminal and run `check-language-support`. You'll see that -en-gb packages have been removed. From ubiquity:

Oct 12 15:52:16 ubuntu ubiquity[9934]: Removing libreoffice-l10n-en-gb (1:6.1.2-0ubuntu1) ...

The installer is supposed to keep these packages, as a result of running `check-language-support -l en_GB --show-installed`. The problem is that the code that is supposed to find en_GB doesn't:

Oct 12 15:48:40 ubuntu localechooser[9037]: info: debian-installer/locale preseeded to 'en_GB.UTF-8' (seen: true)
Oct 12 15:48:40 ubuntu localechooser[9044]: info: Language = 'en'
Oct 12 15:48:40 ubuntu localechooser[9047]: info: line=en;0;US;en_US.UTF-8;;console-setup
Oct 12 15:48:40 ubuntu localechooser[9048]: info: Set debian-installer/language = 'en'
Oct 12 15:48:41 ubuntu localechooser[9049]: info: Default country = 'US'
Oct 12 15:48:41 ubuntu localechooser[9050]: info: Default locale = 'en_US.UTF-8'
Oct 12 15:48:41 ubuntu localechooser[9051]: info: Set debian-installer/consoledisplay = 'console-setup'
Oct 12 15:48:41 ubuntu localechooser[9060]: info: Set debian-installer/country = 'GB'
Oct 12 15:48:41 ubuntu localechooser[9218]: info: Set debian-installer/locale = 'en_GB.UTF-8'
Oct 12 15:48:41 ubuntu localechooser[9229]: info: System locale (debian-installer/locale) = 'en_GB.UTF-8'
Oct 12 15:48:41 ubuntu localechooser[9236]: info: Set debian-installer/language = 'en_GB:en'
Oct 12 15:48:41 ubuntu ubiquity[4546]: debconffilter_done: ubi-timezone (current: ubi-timezone)
Oct 12 15:48:41 ubuntu ubiquity[4546]: Step_before = stepLocation
Oct 12 15:48:41 ubuntu ubiquity[4546]: switched to page usersetup
28520k swap on /target/swapfile. Priority:-2 extents:3 across:744904k FS
Oct 12 15:48:42 ubuntu ubiquity[8547]: File descriptor 3 (pipe:[63593]) leaked on pvs invocation. Parent PID 9406: /bin/sh
Oct 12 15:48:42 ubuntu ubiquity[4546]: debconffilter_done: ubiquity.components.partman_commit (current: ubi-usersetup)
Oct 12 15:48:42 ubuntu /install.py[9568]: keeping packages due to preseeding: icedtea6-plugin openoffice.org
Oct 12 15:48:42 ubuntu /install.py[9568]: keeping language packs for: en_US.UTF-8

See that we set debian-installer/locale to "en_GB.UTF-8", but later on we had forgotten this.

I think it's because the debconf database used in select_language_packs() (instansiated in scripts/install.py) doesn't know about this update. If I modify ubiquity according to the attached patch, install.py sees the right value but then it crashes due to a broken pipe. I don't know how to fix that - help would be appreciated there please on the best way to achieve this.