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.

[wren]

Hi, @yostos! Wow! This seems like it must have been a lot of work, so thank you!

A few notes:

• Can you remove the auto-generated files (pretty much everything inside of the site/ folder)?
• It seems like you might have an auto-formatter on. Can you please keep the PR focused on the relevant changes (example: there are a lot of unrelated changes inside of pyproject.toml file)

Does that make sense? And please let us know if you need help with any of these things.

[ericthebell]

For whatever reason, each of these three example output displays appears to be about 5 characters short of the visual "box" alignment. (The Japanese does have fewer characters than the English, but that doesn't seem to account for the difference.)

[yostos]

When running jrnl with Japanese text, the box formatting actually breaks in the program itself, as shown in the attachment. The documentation merely reflects this current behavior, so the fundamental solution would be to address the handling of multi-byte characters in jrnl's core functionality.

[ericthebell]

Translation review. I reviewed the docs/ja folder and did spot checks throughout a few files, with the PR and original code view side by side:

• contributing.md
• formats.md
• installation.md
• overview.md
• tips-and-tricks.mf
• usage.md

FWIW, the translation looks solid and natural to me.

Notes and caveats

• My Japanese is fluent, but I'm not particularly familiar with any semantic conventions that might apply to technical documentation.
• That said, the translation does a good job of maintaining the casual but clear tone of the English.
• I left one comment where the visual formatting of a box style was misaligned, but I didn't otherwise notice any oddities with code formatting or over-translation (such as code text being translated where it shouldn't be).