OEM setup: user language selection not taken into account

1. Proceed an OEM installation, select English
2. Then, proceed to step 2 (mark for validation)
3. After reboot, on user creation, select a different language like French.
-> Once the user is created and logged in, his session is still in English. Same for login screen.

Notes:
* This is reproducible on 18.04.1.
* This is reproducible as well on 18.04(.0) install without network

Some command output (fr_FR is set, but we can see that langpacks were still removed):
$ locale
LANG=fr_FR.UTF-8
LANGUAGE=
LC_CTYPE="fr_FR.UTF-8"
LC_NUMERIC=fr_FR.UTF-8
LC_TIME=fr_FR.UTF-8
LC_COLLATE="fr_FR.UTF-8"
LC_MONETARY=fr_FR.UTF-8
LC_MESSAGES="fr_FR.UTF-8"
LC_PAPER=fr_FR.UTF-8
LC_NAME=fr_FR.UTF-8
LC_ADDRESS=fr_FR.UTF-8
LC_TELEPHONE=fr_FR.UTF-8
LC_MEASUREMENT=fr_FR.UTF-8
LC_IDENTIFICATION=fr_FR.UTF-8
LC_ALL=
$ check-language-support
firefox-locale-fr gnome-getting-started-docs-fr gnome-user-docs-fr hunspell-en-au hunspell-en-ca hunspell-en-gb hunspell-en-za hunspell-fr hyphen-en-ca hyphen-en-gb hyphen-fr language-pack-fr language-pack-gnome-fr libreoffice-help-en-gb libreoffice-help-fr libreoffice-l10n-en-gb libreoffice-l10n-en-za libreoffice-l10n-fr mythes-en-au mythes-fr thunderbird-locale-en-gb thunderbird-locale-fr wfrench