Hi Hyogo,

As I noticed your website has only one language, so /en/ was indexed from the past.

You can use Joomla Redirect feature or install even a free version of Akeeba Tools - it also has redirect option for the links.

or you can be smart and .... create subpage with alias named "en" and put some english info there.

As Webmaster you know that you can hide that link from Google results - use Google webmaster panel to delete that URL etc.

Update also website sitemap.xml - basics of SEO

Excuse me. https://www.revisiondw.com/index.php/en not,

It was a mistake of https://www.revisiondw.com/index.php/ja/. Can I think of this case in the same way as you said?

Yes, method should be this same.

About /index.php/ In joomla it's used when you don't use rules from .htaccess to rewrite links.

I've got it. Thank you. I'll try it first. If you still don't understand, please ask me.