refactor: cache lowercasing all the locales

[wyattjoh]

This improves the locale normalization method used in several looping functions. This caches the lowercasing of locales so the operation is only performed once.

It also duplicates the logic from the I18nProvider for the more efficient locale handling.

[wyattjoh]

• refactor: cache lowercasing all the locales #74038 👈 (View in Graphite)
• refactor: make locales array immutable #74037
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[ijjk]

Stats from current PR

Default Build (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	vercel/next.js refactor/cache-lowercased-locales	Change
buildDuration	18s	15.7s	N/A
buildDurationCached	14.7s	12.6s	N/A
nodeModulesSize	410 MB	410 MB	⚠️ +66.7 kB
nextStartRea..uration (ms)	474ms	469ms	N/A

Client Bundles (main, webpack)

	vercel/next.js canary	vercel/next.js refactor/cache-lowercased-locales	Change
1187-HASH.js gzip	51.4 kB	51.4 kB	N/A
8276.HASH.js gzip	169 B	168 B	N/A
8377-HASH.js gzip	5.36 kB	5.36 kB	N/A
bccd1874-HASH.js gzip	53 kB	53 kB	N/A
framework-HASH.js gzip	57.5 kB	57.5 kB	N/A
main-app-HASH.js gzip	232 B	235 B	N/A
main-HASH.js gzip	34.1 kB	34.1 kB	N/A
webpack-HASH.js gzip	1.71 kB	1.71 kB	N/A
Overall change	0 B	0 B	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js refactor/cache-lowercased-locales	Change
polyfills-HASH.js gzip	39.4 kB	39.4 kB	✓
Overall change	39.4 kB	39.4 kB	✓

Client Pages

	vercel/next.js canary	vercel/next.js refactor/cache-lowercased-locales	Change
_app-HASH.js gzip	193 B	193 B	✓
_error-HASH.js gzip	193 B	193 B	✓
amp-HASH.js gzip	512 B	510 B	N/A
css-HASH.js gzip	343 B	342 B	N/A
dynamic-HASH.js gzip	1.84 kB	1.84 kB	✓
edge-ssr-HASH.js gzip	265 B	265 B	✓
head-HASH.js gzip	363 B	362 B	N/A
hooks-HASH.js gzip	393 B	392 B	N/A
image-HASH.js gzip	4.49 kB	4.49 kB	N/A
index-HASH.js gzip	268 B	268 B	✓
link-HASH.js gzip	2.35 kB	2.34 kB	N/A
routerDirect..HASH.js gzip	328 B	328 B	✓
script-HASH.js gzip	397 B	397 B	✓
withRouter-HASH.js gzip	323 B	326 B	N/A
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
Overall change	3.59 kB	3.59 kB	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js refactor/cache-lowercased-locales	Change
_buildManifest.js gzip	749 B	746 B	N/A
Overall change	0 B	0 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js refactor/cache-lowercased-locales	Change
index.html gzip	524 B	523 B	N/A
link.html gzip	539 B	538 B	N/A
withRouter.html gzip	520 B	520 B	✓
Overall change	520 B	520 B	✓

Edge SSR bundle Size

	vercel/next.js canary	vercel/next.js refactor/cache-lowercased-locales	Change
edge-ssr.js gzip	128 kB	128 kB	N/A
page.js gzip	204 kB	204 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

	vercel/next.js canary	vercel/next.js refactor/cache-lowercased-locales	Change
middleware-b..fest.js gzip	671 B	666 B	N/A
middleware-r..fest.js gzip	155 B	156 B	N/A
middleware.js gzip	31.2 kB	31.3 kB	N/A
edge-runtime..pack.js gzip	844 B	844 B	✓
Overall change	844 B	844 B	✓

Next Runtimes

	vercel/next.js canary	vercel/next.js refactor/cache-lowercased-locales	Change
523-experime...dev.js gzip	322 B	322 B	✓
523.runtime.dev.js gzip	314 B	314 B	✓
app-page-exp...dev.js gzip	324 kB	324 kB	N/A
app-page-exp..prod.js gzip	128 kB	128 kB	N/A
app-page-tur..prod.js gzip	141 kB	141 kB	N/A
app-page-tur..prod.js gzip	136 kB	136 kB	N/A
app-page.run...dev.js gzip	314 kB	314 kB	N/A
app-page.run..prod.js gzip	124 kB	124 kB	N/A
app-route-ex...dev.js gzip	37.4 kB	37.5 kB	N/A
app-route-ex..prod.js gzip	25.5 kB	25.5 kB	N/A
app-route-tu..prod.js gzip	25.5 kB	25.5 kB	N/A
app-route-tu..prod.js gzip	25.3 kB	25.3 kB	N/A
app-route.ru...dev.js gzip	39 kB	39.1 kB	N/A
app-route.ru..prod.js gzip	25.3 kB	25.3 kB	N/A
pages-api-tu..prod.js gzip	9.69 kB	9.69 kB	✓
pages-api.ru...dev.js gzip	11.6 kB	11.6 kB	✓
pages-api.ru..prod.js gzip	9.68 kB	9.68 kB	✓
pages-turbo...prod.js gzip	21.7 kB	21.7 kB	N/A
pages.runtim...dev.js gzip	27.4 kB	27.5 kB	N/A
pages.runtim..prod.js gzip	21.7 kB	21.7 kB	N/A
server.runti..prod.js gzip	916 kB	916 kB	N/A
Overall change	31.6 kB	31.6 kB	✓

build cache Overall increase ⚠️

	vercel/next.js canary	vercel/next.js refactor/cache-lowercased-locales	Change
0.pack gzip	2.08 MB	2.08 MB	⚠️ +5.41 kB
index.pack gzip	74.5 kB	73.7 kB	N/A
Overall change	2.08 MB	2.08 MB	⚠️ +5.41 kB

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Commit: af340fd

[ijjk]

Tests Passed