Default language loads as English — detect and match browser/system language

[c-kreuz]

Currently, when loading the Couchers site, the default language is set to English regardless of the user’s browser or system settings. This creates a mismatch for non-English speakers and requires manual adjustment to switch to their preferred language.

• Retrieve the browser/system language using standard APIs.
• Map the detected language to available translations in Couchers.
• Use internal fallback routes if the detected language is unsupported.
• Ensure this detection runs only on initial load, respecting any manual language changes made by the user afterward.
• Consider caching the user’s language preference once selected, so it persists across sessions.
• Ensure compatibility with both desktop and mobile browsers.