fix: 404 page incorrectly says Chinese docs are unavailable

[neiljar]

Problem

The custom 404 page (src/theme/NotFound/Content/index.js) always shows:

Chinese documentation is currently unavailable.

This appears on every 404, including English pages. Chinese docs exist, so the message is wrong.

eg.
https://kmesh.net/not-found

Expected

A normal “Page not found” message with Home / Docs links, translated for the current locale.