feat: add translation switcher to website

[3w36zj6]

cf. #233

[3w36zj6]

Could you please do a quick review? @YDX-2147483647 @Its-Just-Nans

[YDX-2147483647]

• English are generally longer than Japanese. Therefore, some expressions take too much space and need to be simplified.

• The translation framework is kinda ad hoc. For example, en-US.tsx is never referenced, so the type checker might neglect it.

• I've just noticed that the footer is missing in TranslationComponent.

  docs/website/src/components/ui/common/Footer.tsx

  Lines 35 to 41 in 4e3161c

  	Translated by{" "}
  	<a
  	href={githubOrganizationUrl}
  	class="text-gray-600 hover:text-gray-800 transition-colors"
  	>
  	Typst Japanese Community
  	</a>

[YDX-2147483647]

Again, English are generally longer than Japanese. To avoid making the layout too tight, I suggest simplifying the expressions.

Perhaps English Version → English and Translation rate → Translated?

[Copilot]

Pull Request Overview

This PR implements a translation switcher system for the website by refactoring the translation architecture to support multiple languages centrally.

• Extracts shared translation types and interfaces into a central index file
• Adds English (en-US) translation alongside existing Japanese (ja-JP) translation
• Updates all import paths to use the new centralized translation structure

Reviewed Changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
website/src/translation/index.tsx	Creates central translation types and exports current language selection
website/src/translation/ja-JP.tsx	Refactors to use shared types and updates import paths
website/src/translation/en-US.tsx	Adds complete English translation implementation
website/src/components/ui/common/*.tsx	Updates import paths to use centralized translation module
website/src/components/ui/*.tsx	Updates import paths to use centralized translation module
website/src/components/templates/*.tsx	Updates import paths to use centralized translation module

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[Copilot]

The message 'This page has been translated into English' is misleading for an English translation file. Since this is the English version, it should indicate the page is in English or original content, not that it was translated into English.

Suggested change

	return <>This page has been translated into English.</>;
	return <>This page is in English.</>;

[3w36zj6]

I also find this a bit odd, but in the first place, the original (English) version shouldn’t need to display translation status, nor should it. Since en-US.tsx is just a sample for i18n support, I think it’s fine to leave it as is.

[YDX-2147483647]

Better, but Footer.tsx was not comitted? Was that intentional?

[3w36zj6]

I accidentally forgot to commit Footer.tsx.

[YDX-2147483647]

Great!