Add Traditional Chinese, Simplified Chinese and Japanese localization

[miguotw]

Adds Traditional Chinese, Simplified Chinese and Japanese to the localization framework in HeyOkay#188, contributing to HeyOkay#162.

Users can select 繁體中文, 简体中文 or 日本語 in Preferences. On first launch, supported Windows UI languages select these catalogs automatically, while saved choices and the English fallback retain the behavior of HeyOkay#188.

• Add complete 105-message catalogs with matching keys and format parameters.
• Register all three languages with an other-only plural rule.
• Add exact Windows LANGID matching for Traditional Chinese (Taiwan, Hong Kong, Macao and Traditional Chinese neutral), avoiding accidental selection on Simplified Chinese systems. Simplified Chinese has a separate catalog and exact LANGIDs for China, Singapore and Simplified Chinese neutral IDs. Japanese uses its primary language ID.
• Extend the existing localization tests and update the English README, contributor guide and changelog.

This PR targets teeti/french-localization and is based on commit 8077b09cd7b410a31fa31fded74ff78a4b7e7d7f. Its diff contains only localization, tests and related documentation; it excludes fork-specific device support, icons, update URLs and versioning.

Validation: python -m unittest discover -s tests — all 502 tests passed on Windows; git diff --check passed. Tests cover catalog parity, formatting parameters, plural rules, language detection, menu selection and localized notifications/states. No separate visual desktop review was performed for this contribution.

The first two commits separate Traditional Chinese/Japanese localization and documentation from added tests. A third commit adds Simplified Chinese and extends the tests and English documentation. They can also be cherry-picked into HeyOkay#188 if preferred.