Handle preferring default locale over accept-lang preferred locale

[ijjk]

This updates to set the NEXT_LOCALE cookie to the default locale when the user prefers a different locale from the default in their accept-language header but visits the default locale path e.g. /en-US with a accept-language preferred header of nl will set the NEXT_LOCALE=en-US header and then redirect to /

x-ref: #17370

[ijjk]

Stats from current PR

Default Server Mode (Decrease detected ✓)

General Overall increase ⚠️

	vercel/next.js canary	ijjk/next.js i18n/handle-prefer-default	Change
buildDuration	12.8s	12.7s	-163ms
nodeModulesSize	63.4 MB	63.4 MB	⚠️ +3.98 kB

Page Load Tests Overall decrease ⚠️

	vercel/next.js canary	ijjk/next.js i18n/handle-prefer-default	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.394	2.387	-0.01
/ avg req/sec	1044.37	1047.46	+3.09
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.252	1.281	⚠️ +0.03
/error-in-render avg req/sec	1997.01	1951.26	⚠️ -45.75

Client Bundles (main, webpack, commons)

	vercel/next.js canary	ijjk/next.js i18n/handle-prefer-default	Change
677f882d2ed8..133b.js gzip	11.1 kB	11.1 kB	✓
framework.HASH.js gzip	39 kB	39 kB	✓
main-3fae807..efe0.js gzip	7.3 kB	7.3 kB	✓
webpack-e067..f178.js gzip	751 B	751 B	✓
Overall change	58.1 kB	58.1 kB	✓

Client Bundles (main, webpack, commons) Modern

	vercel/next.js canary	ijjk/next.js i18n/handle-prefer-default	Change
677f882d2ed8..dule.js gzip	6.9 kB	6.9 kB	✓
framework.HA..dule.js gzip	39 kB	39 kB	✓
main-8ac5189..dule.js gzip	6.29 kB	6.29 kB	✓
webpack-07c5..dule.js gzip	751 B	751 B	✓
Overall change	52.9 kB	52.9 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	ijjk/next.js i18n/handle-prefer-default	Change
polyfills-4b..e242.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	ijjk/next.js i18n/handle-prefer-default	Change
_app-9a0b9e1..b37e.js gzip	1.28 kB	1.28 kB	✓
_error-ed1b0..8fbd.js gzip	3.44 kB	3.44 kB	✓
hooks-89731c..c609.js gzip	887 B	887 B	✓
index-17468f..5d83.js gzip	227 B	227 B	✓
link-409b283..e3ab.js gzip	1.32 kB	1.32 kB	✓
routerDirect..924c.js gzip	284 B	284 B	✓
withRouter-7..c13d.js gzip	284 B	284 B	✓
Overall change	7.73 kB	7.73 kB	✓

Client Pages Modern

	vercel/next.js canary	ijjk/next.js i18n/handle-prefer-default	Change
_app-75d3a82..dule.js gzip	625 B	625 B	✓
_error-4469a..dule.js gzip	2.29 kB	2.29 kB	✓
hooks-cbf13f..dule.js gzip	387 B	387 B	✓
index-b9a643..dule.js gzip	226 B	226 B	✓
link-92d3016..dule.js gzip	1.28 kB	1.28 kB	✓
routerDirect..dule.js gzip	284 B	284 B	✓
withRouter-f..dule.js gzip	282 B	282 B	✓
Overall change	5.37 kB	5.37 kB	✓

Client Build Manifests

	vercel/next.js canary	ijjk/next.js i18n/handle-prefer-default	Change
_buildManifest.js gzip	323 B	323 B	✓
_buildManife..dule.js gzip	329 B	329 B	✓
Overall change	652 B	652 B	✓

Rendered Page Sizes

	vercel/next.js canary	ijjk/next.js i18n/handle-prefer-default	Change
index.html gzip	1 kB	1 kB	✓
link.html gzip	1.01 kB	1.01 kB	✓
withRouter.html gzip	995 B	995 B	✓
Overall change	3.01 kB	3.01 kB	✓

---

Serverless Mode

General Overall increase ⚠️

	vercel/next.js canary	ijjk/next.js i18n/handle-prefer-default	Change
buildDuration	15s	14.7s	-242ms
nodeModulesSize	63.4 MB	63.4 MB	⚠️ +3.98 kB

Client Bundles (main, webpack, commons)

	vercel/next.js canary	ijjk/next.js i18n/handle-prefer-default	Change
677f882d2ed8..133b.js gzip	11.1 kB	11.1 kB	✓
framework.HASH.js gzip	39 kB	39 kB	✓
main-3fae807..efe0.js gzip	7.3 kB	7.3 kB	✓
webpack-e067..f178.js gzip	751 B	751 B	✓
Overall change	58.1 kB	58.1 kB	✓

Client Bundles (main, webpack, commons) Modern

	vercel/next.js canary	ijjk/next.js i18n/handle-prefer-default	Change
677f882d2ed8..dule.js gzip	6.9 kB	6.9 kB	✓
framework.HA..dule.js gzip	39 kB	39 kB	✓
main-8ac5189..dule.js gzip	6.29 kB	6.29 kB	✓
webpack-07c5..dule.js gzip	751 B	751 B	✓
Overall change	52.9 kB	52.9 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	ijjk/next.js i18n/handle-prefer-default	Change
polyfills-4b..e242.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	ijjk/next.js i18n/handle-prefer-default	Change
_app-9a0b9e1..b37e.js gzip	1.28 kB	1.28 kB	✓
_error-ed1b0..8fbd.js gzip	3.44 kB	3.44 kB	✓
hooks-89731c..c609.js gzip	887 B	887 B	✓
index-17468f..5d83.js gzip	227 B	227 B	✓
link-409b283..e3ab.js gzip	1.32 kB	1.32 kB	✓
routerDirect..924c.js gzip	284 B	284 B	✓
withRouter-7..c13d.js gzip	284 B	284 B	✓
Overall change	7.73 kB	7.73 kB	✓

Client Pages Modern

	vercel/next.js canary	ijjk/next.js i18n/handle-prefer-default	Change
_app-75d3a82..dule.js gzip	625 B	625 B	✓
_error-4469a..dule.js gzip	2.29 kB	2.29 kB	✓
hooks-cbf13f..dule.js gzip	387 B	387 B	✓
index-b9a643..dule.js gzip	226 B	226 B	✓
link-92d3016..dule.js gzip	1.28 kB	1.28 kB	✓
routerDirect..dule.js gzip	284 B	284 B	✓
withRouter-f..dule.js gzip	282 B	282 B	✓
Overall change	5.37 kB	5.37 kB	✓

Client Build Manifests

	vercel/next.js canary	ijjk/next.js i18n/handle-prefer-default	Change
_buildManifest.js gzip	323 B	323 B	✓
_buildManife..dule.js gzip	329 B	329 B	✓
Overall change	652 B	652 B	✓

Serverless bundles

	vercel/next.js canary	ijjk/next.js i18n/handle-prefer-default	Change
_error.js	1.06 MB	1.06 MB	✓
404.html	4.34 kB	4.34 kB	✓
hooks.html	3.92 kB	3.92 kB	✓
index.js	1.06 MB	1.06 MB	✓
link.js	1.1 MB	1.1 MB	✓
routerDirect.js	1.1 MB	1.1 MB	✓
withRouter.js	1.1 MB	1.1 MB	✓
Overall change	5.41 MB	5.41 MB	✓

Commit: 5a0f5cb