Add i18n support for English, Russian, and Chinese

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Goal

Add multilingual support to the portfolio.

Languages

• English (en) — default
• Russian (ru)
• Simplified Chinese (zh-CN)

Requirements

• Add an accessible language switcher to the navigation.
• Keep English as the default when no preference has been saved.
• Translate all user-facing content, including navigation, section headings, descriptions, buttons, labels, accessibility text, and metadata where practical.
• Update the document lang attribute when the language changes.
• Persist the selected language locally and restore it on the next visit.
• Preserve the current layout, visual effects, themes, responsive behavior, and generated Projects/Stars marker blocks.
• Ensure generated profile data can be inserted without destroying translated surrounding markup.
• Avoid duplicating effect skins or effect configuration per language.

Acceptance criteria

• A visitor can switch between English, Russian, and Simplified Chinese without reloading the page.
• A first-time visitor sees English.
• The selection survives a reload.
• All three languages work on desktop and mobile in both themes.
• Keyboard navigation and screen-reader labels remain usable.
• Existing site and profile-sync tests remain green; i18n behavior has automated coverage.