`lang: zh-Hant` falls back to Simplified UI strings instead of Traditional

Quarto's subtag-prefix resolution splits lang on - and loads most-general-first:

https://github.com/quarto-dev/quarto-cli/blob/e4b75c1a32902f6d6e01ac62752a5b7d02ec0a12/src/core/language.ts#L56-L133

For lang: zh-Hant this loads _language-zh.yml (Simplified) and then looks for _language-zh-Hant.yml (doesn't exist). Result: Traditional-script readers get Simplified UI strings — wrong script.

Quarto's _language-zh-TW.yml is Traditional in script but uses Taiwan-specific vocabulary (added deliberately in #6512 — OpenCC script conversion followed by manual region-specific corrections). A generic zh-Hant file does not exist.

Design question

How should lang: zh-Hant resolve?

• (a) Fallback rule: if _language-zh-Hant.yml doesn't exist, try _language-zh-TW.yml. Taiwan vocabulary is closer to HK/MO than Simplified is, and this matches Pandoc's posture of shipping one generic zh-Hant file. Could be documented as best-effort until a proper generic file exists.
• (b) Wait for a contributed generic _language-zh-Hant.yml.
• (c) Add _language-zh-Hant.yml as a copy of the TW file. Behaviorally equivalent to (a) but less honest about regional scope.

Repro

---
lang: zh-Hant
format: html
---

Inspect generated UI strings (TOC title, "Copy to clipboard" tooltip, section titles in HTML article). They render in Simplified Chinese.

From quarto-dev/quarto-cli#14409. Related: #14416.

Start in src/core/language.ts at the subtag-prefix resolution described in the issue, then inspect _language-zh.yml and _language-zh-TW.yml. Reproduce the lang: zh-Hant example and compare the generated UI strings. Done means the selected resolution behavior is agreed, documented, and the repro renders Traditional Chinese strings without regressing existing locale behavior.