fix: Language detection by validating URL path segments

[wo-o29]

📝 Key Changes

Fixes: #599

Fixed language detection bug in navigation sidebar where non-language URL segments were incorrectly treated as language codes.

Problem

• On paths like https://frontend-fundamentals.com/today-i-learned/post/D_kwDONfHk5s4AitBU, the lang prop was incorrectly set to "post"
• This caused navigation links to be malformed (e.g., clicking on a11y would navigate to /a11y/post instead of /a11y)

Solution

• Extract language segment from location.pathname and validate against SUPPORTED_LANGUAGES (["ko", "en", "ja", "zh-hans"])
• If valid language code is found, use it; otherwise default to "ko"
• Implemented extractLanguageCode() function for centralized validation logic

🖼️ Before and After Comparison

Before	After

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[milooy]

👍