docs: add README_zh-TW.md translation

[doggy8088]

Translate README.md to Traditional Chinese (README_zh-TW.md) and update language switcher links in other README files. Relates to #2870

[CLAassistant]

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[Copilot]

Pull request overview

Adds a Traditional Chinese (zh-TW) translation of the project README and updates the language switcher links across existing README translations so readers can navigate to the new document (and, in several READMEs, to the Portuguese translation as well).

Changes:

• Add README_zh-TW.md as a Traditional Chinese translation of the main README.
• Add a “繁體中文” link to the language switcher in README.md and other translated READMEs.
• Normalize language switcher formatting in several translated READMEs and add a README_pt.md link where it was missing.

Review notes (actionable):

• In README_zh-TW.md, the Nicolas Le Cam entry uses Github instead of GitHub in the link label (spelling/capitalization inconsistency). This should be corrected.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
README.md	Adds the Traditional Chinese link to the language switcher.
README_zh.md	Adds Traditional Chinese + Portuguese links and normalizes the switcher formatting.
README_zh-TW.md	New Traditional Chinese translation of the main README.
README_pt.md	Adds the Traditional Chinese link to the language switcher.
README_ko.md	Adds Traditional Chinese + Portuguese links and normalizes the switcher formatting.
README_ja.md	Adds Traditional Chinese + Portuguese links and normalizes the switcher formatting.
README_fr.md	Adds Traditional Chinese + Portuguese links and normalizes the switcher formatting.
README_es.md	Adds Traditional Chinese + Portuguese links and normalizes the switcher formatting.