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

[goktugozkanmd]

Refreshed this branch against current main (64f4f5a) without dropping the existing commit, then revalidated the issue #745 path.

What is now covered:

• keeps Language.value/ISO-639-3 lookup compatibility;
• preserves the FLORES ISO-15924 suffix in LanguageWithScript;
• restores zho_Hant with Traditional Chinese literals while other script variants fall back to their base-language literals;
• retains regression coverage for duplicate-code normalization and prompt rendering.

Validation on the refreshed head e234530:

pytest -q tests/unit/utils/test_language.py tests/unit/tasks/test_flores200.py tests/unit/tasks/templates/test_multichoice.py
9 passed

ruff check <6 changed Python files>
All checks passed!

ruff format --check <6 changed Python files>
6 files already formatted

The PR currently has no hosted checks reported on the fork branch, so these are the exact local results. It is ready for maintainer direction/review.