Ubiquity does not set "Switching to another layout" shortcut for secondary language

[Norbert]

Steps to reproduce:
1. Start Ubuntu MATE 18.04.5 LTS installer
2. Specify Russian language on the first page of installer
3. Wait installer to finish

Expected results:
* the keyboard layout switcher shows two layouts - *en* and *ru*, reacts on <Alt>+<Shift> hotkey

Actual results:
* the keyboard layout switcher is not shown, does not react on <Alt>+<Shift> hotkey

Note: the contents of keyboard configuration files is the following:

```
$ cat /etc/default/keyboard
# KEYBOARD CONFIGURATION FILE

# Consult the keyboard(5) manual page.

XKBMODEL="pc105"
XKBLAYOUT="us,ru"
XKBVARIANT=","
XKBOPTIONS="grp_led:scroll"

BACKSPACE="guess"

```

so the Russian keyboard layout was added, but <Alt>+<Shift> is not defined.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubiquity (not installed)
ProcVersionSignature: Ubuntu 5.4.0-42.46~18.04.1-generic 5.4.44
Uname: Linux 5.4.0-42-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.16
Architecture: amd64
CurrentDesktop: MATE
Date: Mon Aug 10 21:04:09 2020
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu-mate.seed boot=casper only-ubiquity quiet splash ---
InstallationDate: Installed on 2020-08-10 (0 days ago)
InstallationMedia: Ubuntu-MATE 18.04.5 LTS "Bionic Beaver" - Release amd64 (20200806.1)
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)

[Norbert (nrbrtx)]

Ubuntu MATE 16.04 LTS (now EOL) sets "grp:alt_shift_toggle" correctly.

[Ubuntu QA Website (ubuntuqa)]

This bug has been reported on the Ubuntu ISO testing tracker.

A list of all reports related to this bug can be found here:
http://iso.qa.ubuntu.com/qatracker/reports/bugs/1891070

[Norbert (nrbrtx)]

Russian setup on impish leads to only `us` layout configured in /etc/default/keyboard .

[Norbert (nrbrtx)]

Still happens on Jammy.