Chinese (zh) language option needs to be enabled in the interface

[novusnota]

• Middleware and routing should enable language switching (next.config.js + new middleware.js in the root)
• All the pages should get their empty Chinese counterparts, so that Chinese translators may start filling those in

Until then i18n support in routing is only implicitly enabled (one may add a /en prefix to any routes and they would continue working smoothly).

This issue is made as a follow-up to #79 in attempt to decrease the size of individual PRs.

[novusnota]

Resolved in tact-lang/tact#880, alongside with the migration of the docs to the compiler repo