What is the reason why the basic commands are not displayed in Japanese in man?

environment

Vagrant + Ubuntu 16.04.5 LTS

Correspondence

When I display the top page of man, the Japanese manual is displayed properly.

$man man

For some reason, such basic commands are displayed in English.

$man ls
$man cat

** ja_JP.utf8 ** is set properly in the locale.

$locale -a

Japanese package of man is included.

$apt list --installed | grep manpages-ja
manpages-ja/xenial,xenial,now 0.5.0.0.20140515+dfsg-2 all [Installation complete]
manpages-ja-dev/xenial,xenial,now 0.5.0.0.20140515+dfsg-2 all [Installation complete]

Looking at whether there is a package for each command of man, cat and ls do not exist. why?

$ls -l /usr/share/man/ja/man1/{cat,ls,man}.1.gz
ls: '/usr/share/man/ja/man1/cat.1.gz'Cannot access:There is no such file or directory
ls: '/usr/share/man/ja/man1/ls.1.gz'Cannot access:There is no such file or directory
-rw-r--r--1 root root 15K November 7 2015/usr/share/man/ja/man1/man.1.gz

For the time being, I decided to install the latest version.

$sudo apt install -y manpages-ja manpages-ja-dev

It seems to be installed.

$ls -l /usr/share/man/ja/man1/{cat,ls,man}.1.gz
-rw-r--r-- 1 root root 1.4K December 16 2015/usr/share/man/ja/man1/cat.1.gz
-rw-r--r-- 1 root root 4.0K December 16 2015/usr/share/man/ja/man1/ls.1.gz
-rw-r--r--1 root root 15K November 7 2015/usr/share/man/ja/man1/man.1.gz

Eventually, Japanese is displayed.

$man ls
$man cat