Fix classroom generation language normalization for non-English locales

[GRIZZZZZLY]

Summary

This PR fixes a language handling bug in classroom generation where any non-English language could be coerced to zh-CN.

Problem

The generation pipeline treated en-US as a special case and defaulted other languages to Chinese. That caused incorrect output language behavior for languages like Russian.

Changes

• adds proper normalization for supported generation languages
• adds explicit support for ru-RU
• removes hardcoded assumptions that only zh-CN and en-US are valid generation languages
• improves prompt-level language enforcement in classroom generation

Why this matters

This fixes a real generation bug and improves the multilingual architecture of the generation pipeline.

Notes

This PR focuses on generation correctness, not full UI localization.

[GRIZZZZZLY]

Hi! I fixed the formatting issue that previously caused CI to fail on this PR.\n\nThe latest workflow run now shows action_required rather than a code failure, so it looks like GitHub Actions may need maintainer approval to run for this fork PR.\n\nCould someone please approve and run the workflows for this PR? Thanks!