Add Japanese translations and implement I18N support

[yostos]

This pull request adds Japanese translations for the documentation and implements I18N support for the project.

Intended changes:

• Add Japanese translations for main documentation files
• Implement an I18N framework to support multiple languages
• Update mkdocs.yml to enable multi-language support

Motivation:
To make the project more accessible to Japanese-speaking users and to establish a framework for easy addition of other languages in the future.

There are no directly related issues, but this change aligns with the project's goal of international accessibility.

Example of changes:

• New 'ja/' directory added with translated .md files
• mkdocs.yml updated to include language selection options

Checklist

• I have read the contributing doc.
• I have included a link to the relevant issue number.
• I have checked to ensure there aren't other open pull requests
  for the same issue.
• I have written new tests for these changes, as needed.