client: fix i18next config to generate i18n data correctly

[virusbb001]

I tried to create JSON data by running npm run translate in the repository's root directory as per https://github.com/leanprover-community/lean4game/blob/main/doc/translation-interface.md, but instead of being created in client/public/locales/{language}/translation.json as it should have been, it was created as an empty object in client/client/public/locales/{language}/translation.json. This is because when I ran it with npm --workspace client, client/ became the working directory, and the i18next settings were also treated as relative to that directory.

[virusbb001]

When regenerating for other languages ​​(such as en and de), some translations were lost. This pull request needs to be corrected.

[virusbb001]

Since keySeparator was set to false, I changed the JSON to the corresponding format.
Also, after updating the translation data, I noticed some unexpected deletions in some keys, so I set removeUnusedKeys to false. I believe it can be safely set back to true after updating the translation data and resolving the warnings generated by i18next-scanner.
For newly created translation data, you can set it to true in the following way:

removeUnusedKeys: function(lng, ns, key) {
  return lng === "ja";
}

I am not familiar with either language, so I have not made any changes that would require changing the values ​​in the translation data.