Update messages_ja_JP.properties

[tkymmm]

Description of Changes

Please provide a summary of the changes, including:

• What was changed
• Why the change was made
• Any challenges encountered

Closes #(issue_number)

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Checklist

General

• I have read the Contribution Guidelines
• I have read the Stirling-PDF Developer Guide (if applicable)
• I have read the How to add new languages to Stirling-PDF (if applicable)
• I have performed a self-review of my own code
• My changes generate no new warnings

Documentation

• I have updated relevant docs on Stirling-PDF's doc repo (if functionality has heavily changed)
• I have read the section Add New Translation Tags (for new translation tags only)

UI Changes (if applicable)

• Screenshots or videos demonstrating the UI changes are attached (e.g., as comments or direct attachments in the PR)

Testing (if applicable)

• I have tested my changes locally. Refer to the Testing Guide for more details.

[github-actions]

🚀 Translation Verification Summary

🔄 Reference Branch: main-branch-messages_en_GB.properties

📃 File Check: messages_ja_JP.properties

1. Test Status: ✅ Passed
2. Test Status: ✅ Passed
3. Test Status: ✅ Passed

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✅ Overall Check Status: Success

Thanks @tkymmm for your help in keeping the translations up to date.