Error Message:

JInstaller::install: The language file 'ja-JP.plg_system_customheadtag.ini' could not be installed because the Administrator language package for 'ja-JP' has not been installed or does not exist.

This message does not have any problem. Because there was not a language folder, only a language of ja-JP was not installed.

The installation of this extension is completed.

日本語のランゲージと英語のランゲージを一緒にしているのですが、別にした方が良いのかな？

様子をみよう・・・。