Thank you for your reply.

> The fact that Mozc isn't available is indeed an issue. It sounds to me
> as the same issue as the one reported at bug #1786344. Do you agree on
> that?

It seems to me this bug is a little different from the bug #1786344.
Comment #10 of the bug #1786344 you wrote could be related this bug,
but other comments may be related to other problem on inputting chinese.

> Besides that, it's unclear to me how your other observations are
> problems. If you think they are, please let us know what you think
> should be the expected behavior in the various examples.

This bug occurs only after maybe-ubiquity mode(ubiquity --greeter).
According to the bug #1766047, it's impossible to boot in maybe-ubiquity mode
when booting in UEFI. So this bug occurs when booting in legacy BIOS.

When choosing 日本語(Japanese) on isolinux menu and select
インストールせずにUbuntuを試してみる(Try Ubuntu without installing),
I can use Mozc without problems on live session.
dconf shows ('ibus', 'mozc-jp') for input-sources as follows.
This is totally expected behavior.
--
ubuntu@ubuntu:~$ cat /proc/cmdline
file=/cdrom/preseed/ubuntu.seed boot=casper initrd=/casper/initrd quiet splash --- debian-installer/language=ja keyboard-configuration/layoutcode?=jp keyboard-configuration/modelcode?=jp106
ubuntu@ubuntu:~$ dconf dump /org/gnome/desktop/input-sources/
[/]
sources=[('xkb', 'jp'), ('ibus', 'mozc-jp')]
xkb-options=['lv3:ralt_switch']
--

But when waiting for ubiquity greeter dialog, select 日本語(Japanese), click Ubuntuを試す(Try Ubuntu),
I can't use Mozc on live session.
no ('ibus', 'mozc-jp') for input-sources and rarely used Japanese input sources are listed as follows.
--
ubuntu@ubuntu:~$ cat /proc/cmdline
file=/cdrom/preseed/ubuntu.seed boot=casper initrd=/casper/initrd quiet splash --- maybe-ubiquity
ubuntu@ubuntu:~$ dconf dump /org/gnome/desktop/input-sources/
[/]
sources=[('xkb', 'jp'), ('xkb', 'nec_vndr/jp'), ('xkb', 'jp+OADG109A'), ('xkb', 'jp+dvorak')]
xkb-options=@as []
--

I tried selecting other languages, the result is as follows.

Select Simplified Chinese on isolinux menu:
--
ubuntu@ubuntu:~$ cat /proc/cmdline
file=/cdrom/preseed/ubuntu.seed boot=casper initrd=/casper/initrd quiet splash --- locale=zh_CN keyboard-configuration/layoutcode?=cn
ubuntu@ubuntu:~$ dconf dump /org/gnome/desktop/input-sources/
[/]
sources=[('xkb', 'cn'), ('ibus', 'libpinyin')]
xkb-options=['lv3:ralt_switch']
--
I don't know this is ideal settings for Chinese or not, but it includes ('ibus', 'libpinyin') anyway.


Select Simplified Chinese on Ubiquity Greeter:
--
ubuntu@ubuntu:~$ cat /proc/cmdline
file=/cdrom/preseed/ubuntu.seed boot=casper initrd=/casper/initrd quiet splash --- maybe-ubiquity
ubuntu@ubuntu:~$ dconf dump /org/gnome/desktop/input-sources/
[/]
sources=[('xkb', 'cn')]
xkb-options=@as []
--
Only [('xkb', 'cn')] is. This is the same as comment #10 of bug #1786344.


Select English on isolinux menu:
--
ubuntu@ubuntu:~$ cat /proc/cmdline
file=/cdrom/preseed/ubuntu.seed boot=casper initrd=/casper/initrd quiet splash ---
ubuntu@ubuntu:~$ dconf dump /org/gnome/desktop/input-sources/
[/]
sources=[('xkb', 'us')]
xkb-options=@as []
--


Select English on Ubiquity Greeter:
--
ubuntu@ubuntu:~$ cat /proc/cmdline
file=/cdrom/preseed/ubuntu.seed boot=casper initrd=/casper/initrd quiet splash --- maybe-ubiquity
ubuntu@ubuntu:~$ dconf dump /org/gnome/desktop/input-sources/
[/]
sources=[('xkb', 'us'), ('xkb', 'au'), ('xkb', 'cm'), ('xkb', 'gb')]
xkb-options=@as []
--
4 input sources are listed.


Select Svenska on isolinux menu:
--
ubuntu@ubuntu:~$ cat /proc/cmdline
file=/cdrom/preseed/ubuntu.seed boot=casper initrd=/casper/initrd quiet splash --- debian-installer/language=sv keyboard-configuration/layoutcode?=se
ubuntu@ubuntu:~$ dconf dump /org/gnome/desktop/input-sources/
[/]
sources=[('xkb', 'se')]
xkb-options=['lv3:ralt_switch']
--


Select Svenska on Ubiquity Greeter:
--
ubuntu@ubuntu:~$ cat /proc/cmdline
file=/cdrom/preseed/ubuntu.seed boot=casper initrd=/casper/initrd quiet splash --- maybe-ubiquity
ubuntu@ubuntu:~$ dconf dump /org/gnome/desktop/input-sources/
[/]
sources=[('xkb', 'se'), ('xkb', 'se+dvorak'), ('xkb', 'se+mac'), ('xkb', 'se+nodeadkeys')]
xkb-options=@as []
--

I don't know much about other than Janpanese, but I can confirm input-souces settings