Document language/internationalization variables and functions

[Copilot]

This PR completes the documentation of AssemblyLine's language/internationalization system by adding comprehensive coverage of variables and functions defined in language.py and al_language.yml.

Changes Made

Added Assembly Line language system variables to magic_variables.md:

• enable_al_language - Controls whether Assembly Line language system is active
• al_user_default_language - Default language code when user hasn't selected one
• al_interview_languages - List of supported language codes for the interview
• al_user_language - Currently selected language for the user
• al_change_language (event) - Event triggered when user switches languages

Enhanced translation.md documentation:

• Added comprehensive section explaining the al_change_language event
• Documented how the event works internally and when it's triggered
• Provided example of how to add custom logic during language switches
• Improved cross-references to the special variables page

Improved cross-references between documentation pages:

• Added links between translation.md and magic_variables.md
• Clear distinction between general localization variables (AL_DEFAULT_LANGUAGE) and Assembly Line-specific ones
• Better organization of language-related documentation

Before/After

Previously, the Assembly Line language variables were only documented in the translation guide. Now they're properly categorized in the special variables reference alongside other runtime variables, making them easier to discover and understand in context.

The al_change_language event was only briefly mentioned before - now it has comprehensive documentation explaining its purpose, behavior, and how developers can extend it.

All language/internationalization variables and functions from the AssemblyLine framework are now comprehensively documented with proper cross-references between related pages.

Fixes #173.

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