Add product translation specifications

This MR establishes the product translation framework by adding:

What's Added

• Product Translation Module configuration in chat rules for localization group requests
• Core style guide(specifications/product/core/gitlab-translation-styleguide.md) with comprehensive translation principles, workflow, and quality standards
• Japanese style guide (specifications/product/languages/ja-jp/styleguide.md) with language-specific guidelines for product translations

Key Features

• Complete translation workflow (resource loading → execution → validation)
• Terminology management with termbase.csv integration - Cultural adaptation guidelines for Japanese business context
• Technical precision rules for code, links, and identifiers
• Quality assurance checklist with success metrics

Updated

• Marketing module scope refined to `digital-experience` group This complements the existing marketing translation specifications and provides dedicated resources for product content localization.