The i18n decision you make before translating: URL shape, and why ?lang= cannot be undone

The query parameter loses because two systems read the URL and nothing else, and neither of them is part of the application. Search engines fold `?lang=zh` and `/` into one page, so the Chinese version arrives as duplicate content ^[6]. People then share the link without the parameter attached, which means the shared copy is the default language whatever the recipient speaks ^[8].

On a server-rendered deployment none of this is terminal. You add the query to the cache key, or you rewrite the request before it reaches the cache. A static export removes both, and the same missing runtime shows up in the writeup as its own separate bug: `redirect()` on the root path did nothing, because `output: 'export'` has no server to execute it and Next.js skipped it in silence ^[13]. The replacement was an inline script reading `navigator.language` and calling `window.location.replace`, with a `noscript` meta refresh behind it ^[14]. That is the shape of every recovery path when the only code you can run is code the browser downloads.

Here is the arithmetic the pros-and-cons lists skip. The site is 100+ pages [c1b]. Put the locale in the path and `generateStaticParams` prerenders 200+ URLs at two languages, each one its own object in the CDN ^[11][2]. Keep the locale in the query and you have roughly 100 objects being asked to serve an audience split about evenly between China and everywhere else ^[2][2], with the author reporting that the copy without the query is the one that wins the cache ^[7]. Nothing in the application is broken. The edge is simply answering a question it was never given.

The other four failures in the same writeup are cheap by comparison. Two layouts each rendering `html` and `body` produced nested documents and doubled hydration ^[12]. `params` arrived as a Promise, so `generateMetadata` read `locale` as undefined and shipped English titles under `/zh-CN/` ^[15]. A `setLocale` that only touched React state changed the URL and left the content alone ^[17]. All of those fail loudly, on first load, on the machine that built them.

The URL decision fails somewhere else: in an index you do not control and a cache you do not own, on a delay. It is also the one decision the tooling cannot express without you. `alternates.languages` and a per-locale canonical only mean something when there is a distinct URL to point them at ^[16]. Pick the query string and you have built a site whose language variants cannot be described to the only two consumers that matter.