Update Language Flags to ISO 639-1 Standard

[kassyray]

Standardize all language flags in pipeline to use ISO 639-1 two-letter language codes (e.g., en, fr, es) for interoperability with modern translation systems and APIs.

• Ensures compatibility with AI translation services (OpenAI, Azure, Google Translate, etc.).
• Aligns with standard locale codes used in web and OS internationalization.
• Simplifies directory names and configuration references.
• Prepares the system for multi-language expansion (e.g., en, fr, es, ar).

Proposed Changes

1. Folder & File Naming

• Rename existing directories:
  json_english/ → json_en/
json_french/ → json_fr/

• Update all path references in scripts and configs.

2. Script Inputs

• Update CLI arguments to accept only ISO 639-1 codes.
• Add validation and descriptive error messages.

3. Configuration Files

• Update YAML/Typst config keys (language: en) instead of full names.
• Update README.md and onboarding guides with the two-letter standard.
• Include a link to the ISO 639-1 list.

Examples of SDKs or AI tools that accept ISO 639-1 codes include Azure AI Translator.