Hello pilou2!
I had Saturday and Sunday off and Monday to get some work done.

I was able to do the 'myadmin/includes/languages/japanese/images/buttons/button_display.gif' by uploading it again.

I have experimented with various test pages since then, and when I add 'mysql_japanese_config.sql', the Japanese combo box appears, but I get an error on the storefront page.' This page is not working, the domain cannot process this request at this time. HTTP ERROR 500'
If I change the PHP language setting on the server from 8 to 7.4 module mode, the storefront page seems to display. However, it was an incomplete display.

Then I read your reply and tried the following
The "Japanese" folder and the "lang.japanese.php" file in the "ZC158LangPack_JP\includes\languages", I uploaded them to the "/Mydomain/Myfolder/includes/languages" folder on my server.

A combo box appeared where I could switch between Japanese and English. I was able to go to the payment and shipping module tabs and they displayed correctly. Excellent.
' mysql_japanese_config.sql' and the faulty front page now displays correctly.
However, the top page and other displays are now probably from your own page.
The payment methods etc. are also probably the same as on your page.
This helped me understand that I could change the storefront text by changing the text in the folders.

I have not uploaded the Japanese pack for the module, but it now displays correctly. I am going to try to manipulate it in various ways.