docs(react): document the Pages Router i18n config mismatch warning

[moss-bryophyta]

What

Documents the build-time warning added by gt#2005, shipped in gt-next@11.1.4 (published 2026-07-30).

When next.config.ts's i18n.locales / i18n.defaultLocale disagree with gt.config.json, gt-next now prints a warning at build time. Nothing in the docs mentioned it.

Why it matters

Both Pages Router pages already show the correct pattern — importing the values out of gt.config.json so the two configs cannot drift:

const nextConfig: NextConfig = {
  i18n: {
    locales: gtConfig.locales,
    defaultLocale: gtConfig.defaultLocale,
  },
};

Neither page explained why that import matters. A reader who hardcodes the arrays instead (a natural thing to do) gets a warning with no docs to search for. This adds the missing rationale and names the warning.

Changes

• react/(frameworks)/nextjs/pages-router-middleware.mdx — one paragraph after the config block explaining that importing keeps the two configs in sync, what the warning says, and that the locale comparison is order-independent.
• react/nextjs-pages-router-quickstart.mdx — one clause noting the values should be imported rather than repeated, and that gt-next warns when they disagree. Kept short since this page already links to the routing page for details.

I left pages-router-static-site-generation.mdx alone — it already says to import from gt.config.json and links to the routing page for complete setup, so a third copy would be redundant.

Verification

Behavior confirmed against the published gt-next@11.1.4, not just main:

gt-next (plugin) Warning: Next.js internationalized routing does not match
the GT config file because next.js may select a locale that GT is not
configured to translate. ...

Order-independence and the defaultLocale-prepend normalization are confirmed in getNextI18nConfigMismatches / haveSameLocales in packages/next/src/config.ts. The check is deliberately scoped to file-loaded config only — inline props passed to withGTConfig go through the existing conflict/merge paths and do not trigger this warning, so I did not claim otherwise in the docs.

All five validators pass, and pnpm run build:content succeeds (328+ paths).

Related

While verifying the exact warning text I found the shared diagnostic formatter lowercases proper nouns, rendering because next.js may select.... Filed upstream as gt#2016. The docs here deliberately describe the warning's meaning rather than quoting the mangled string, so this PR stays correct either way.