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

[Norbert]

Steps to reproduce:
1. Start Ubuntu MATE 20.04 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"
XKBVARIANT=""
XKBOPTIONS=""

BACKSPACE="guess"

```

so the Russian keyboard layout were not added.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: ubiquity (not installed)
ProcVersionSignature: Ubuntu 5.4.0-21.25-generic 5.4.27
Uname: Linux 5.4.0-21-generic x86_64
ApportVersion: 2.20.11-0ubuntu21
Architecture: amd64
CurrentDesktop: MATE
Date: Wed Apr 1 23:17:00 2020
InstallCmdLine: file=/cdrom/preseed/ubuntu-mate.seed initrd=/casper/initrd quiet splash --- maybe-ubiquity
InstallationDate: Installed on 2020-04-01 (0 days ago)
InstallationMedia: Ubuntu-MATE 20.04 LTS "Focal Fossa" - Beta amd64 (20200401)
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)

[Norbert (nrbrtx)]

Second time the installed system have `ru` and `us` layouts, but the <Alt>+<Shift> is not defined.

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 keyboard layout switcher is not defined here.

There is something wrong with keyboard layout settings by installer.

[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/1870219

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Norbert (nrbrtx)]

Exists in beta. Language used does not really matter.

[Norbert (nrbrtx)]

For Xubuntu the results are worse - keyboard layout switcher applet after adding does not detect `ru` / Russian language which is defined in `/etc/default/keyboard`.