Add script-aware language tags for FLORES 200

[goktugozkanmd]

Summary

• Add Script, LanguageWithScript, and language_from_tag() to preserve script information from dataset tags such as zho_Hant, while Language.value keeps the base ISO 639 code
• Add script-aware translation literal lookup with fallback to the base language
• Add Traditional Chinese template literals and re-enable zho_Hant in FLORES 200

Fixes #745.

Tests

• ruff check and ruff format --check pass on the changed files
• pytest tests/unit/prompt tests/unit/tasks tests/unit/utils/test_language.py — 67 passed