Even if you set LANG=ja_JP.UTF-8, if the locale definition does not exist you will see error messages like the following.
locale: Cannot set LC_CTYPE to default locale: No such file or directory locale: Cannot set LC_MESSAGES to default locale: No such file or directory locale: Cannot set LC_ALL to default locale: No such file or directory
Add the Japanese locale with the locale-gen command.
$ sudo locale-gen ja_JP.UTF-8
You can see that it has been added.
$ locale -a C C.UTF-8 POSIX en_US.utf8 ja_JP.utf8