Improve Case-Insensitive Key Comparison and Path Normalization in Language Properties Check Script

[Ludy87]

Description of Changes

Please provide a summary of the changes, including:

• Updated key comparison logic in update_missing_keys function to be case-insensitive by converting keys to lowercase before comparison.
• Introduced os.path.normpath for file path normalization to improve cross-platform compatibility.
• Replaced direct usage of file_path with file_normpath in security checks, file size validation, and duplicate key detection to ensure consistent path handling.

These changes improve the robustness and maintainability of the script, ensuring more accurate language property checks while enhancing security validation.

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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.