Drupal Commerce uses the Address module for handling addresses.
In Address fields, Japanese prefectures correspond to Administrative area (e.g. State or Province).
I just installed the Address module in a Japanese language Drupal 10 site and the prefectures were translated correctly by default. So, it seems you failed to get the translations when you installed the module.
Usually, contrib module translations are bundled as PO files with the module. However, for the administrative divisions in the Address module, the commerceguys/addressing library is used, which provides its own translations.
There are docs for modifying the translations, but since the prefecture should already be translated, I would do the following:
- Make sure that you installed Commerce/the Address module with composer.
- Check the translation settings at
/admin/config/regional/translate/settings
. Make sure that the translation source is Drupal translation server and local files.
- If the site has multiple languages, check the language settings at
/admin/config/regional/language/detection
and ensure that the language is being correctly set to Japanese for the logged-in account.