A lot of pages with Japanese content are indexed in google but when I go there it says page not found

I answered a question from support.google.com/webmasters/ i think this answer will help.

I am facing the same issue, I fixed the site maybe google will re-crawl and correct URLs in its search, I fixed the site 2 days back but still seeing Japanese texts are there on google search results just like yours.

How I fixed the site:

Somehow the site was hacked, so I worked for making the site clean, so by default Google will see the clean site now. It is probably the sitemap google is getting from your site and hacker hacked the sitemap, so I cleaned the sitemap, I use Yoast SEO. You can check your sitemap I guess you will find those unknown links in sitemap also.

1) Use security plugins to check the site, most probably WordPress in my case the core files were affected so it is a bit easy to find hacking codes compared to clean WordPress installation, the security plugin will point hack codes and replace those files and check the site.

2) After cleaning the site, check if the sitemap is clean as expected IF NOT take a backup of the site using all in one migration plugin and install the backup in another hosting, and again check the site with a security plugin and Yoast SEO sitemap style.

I did the above steps and found out my sitemap is clean and all unknown links are gone. So I hope Google will delete those 404 links from google search and re-crawl my site.

security plugin used to check affected files:
Sucuri Security – Auditing, Malware Scanner and Security Hardening

bonus: make the site’s security very very tight so no one can change or write on files and directories to avoid future hack. Whenever you see errors in search console re-check your sitemap to check if the hacking is done again or not, I check daily after the hack, hope this process will work.