feat: remove nuxt i18n for in house i18n for web (#5131)

* feat: remove nuxt i18n for in house

* cleanup: remove old nuxt/i18n patch

* prepr

---------

Co-authored-by: Prospector <6166773+Prospector@users.noreply.github.com>

apps/frontend/src/plugins/i18n.ts

@@ -0,0 +1,112 @@
+import {
+	type CrowdinMessages,
+	I18N_INJECTION_KEY,
+	type I18nContext,
+	LOCALES,
+	transformCrowdinMessages,
+} from '@modrinth/ui'
+import IntlMessageFormat from 'intl-messageformat'
+import { LRUCache } from 'lru-cache'
+
+const DEFAULT_LOCALE = 'en-US'
+
+const localeModules = import.meta.glob<{ default: CrowdinMessages }>('../locales/*/index.json', {
+	eager: false,
+})
+
+const messageCache = new LRUCache<string, Record<string, string>>({ max: 10 })
+const formatterCache = new LRUCache<string, IntlMessageFormat>({ max: 1000 })
+const loadingPromises = new Map<string, Promise<void>>() // Dedupe concurrent loads
+
+async function loadLocale(code: string): Promise<void> {
+	if (messageCache.has(code)) return
+
+	// Dedupe concurrent requests for the same locale
+	const existing = loadingPromises.get(code)
+	if (existing) return existing
+
+	const promise = (async () => {
+		const loader = localeModules[`../locales/${code}/index.json`]
+		if (!loader) return
+		const raw = await loader()
+		messageCache.set(code, transformCrowdinMessages(raw.default))
+	})()
+
+	loadingPromises.set(code, promise)
+	try {
+		await promise
+	} finally {
+		loadingPromises.delete(code)
+	}
+}
+
+function parseAcceptLanguage(header: string): string | null {
+	try {
+		for (const lang of header
+			.split(',')
+			.map((l) => l.split(';')[0]?.trim())
+			.filter(Boolean)) {
+			const exact = LOCALES.find((loc) => loc.code === lang)
+			if (exact) return exact.code
+			const prefix = LOCALES.find((loc) => loc.code.startsWith(lang.split('-')[0] + '-'))
+			if (prefix) return prefix.code
+		}
+	} catch {
+		// Malformed header, ignore
+	}
+	return null
+}
+
+export default defineNuxtPlugin({
+	name: 'i18n',
+	enforce: 'pre',
+	async setup(nuxtApp) {
+		// ONLY locale needs request-scoping (what language this request uses)
+		const locale = useState<string>('i18n-locale', () => DEFAULT_LOCALE)
+
+		function t(key: string, values?: Record<string, unknown>): string {
+			const msg = messageCache.get(locale.value)?.[key] ?? messageCache.get(DEFAULT_LOCALE)?.[key]
+			if (!msg) return key
+			if (!values || Object.keys(values).length === 0) return msg
+
+			const cacheKey = `${locale.value}:${msg}`
+			let formatter = formatterCache.get(cacheKey)
+			if (!formatter) {
+				formatter = new IntlMessageFormat(msg, locale.value)
+				formatterCache.set(cacheKey, formatter)
+			}
+			try {
+				return formatter.format(values) as string
+			} catch {
+				return msg
+			}
+		}
+
+		async function setLocale(newLocale: string): Promise<void> {
+			if (!LOCALES.some((l) => l.code === newLocale)) return
+			await loadLocale(newLocale)
+			locale.value = newLocale
+			useCookie('locale', { maxAge: 31536000, path: '/' }).value = newLocale
+		}
+
+		// Detect initial locale (cookie > Accept-Language > default)
+		const cookieLocale = useCookie('locale').value
+		let detectedLocale = DEFAULT_LOCALE
+		if (cookieLocale && LOCALES.some((l) => l.code === cookieLocale)) {
+			detectedLocale = cookieLocale
+		} else if (import.meta.server) {
+			const acceptLang = useRequestHeaders(['accept-language'])['accept-language']
+			if (acceptLang) {
+				detectedLocale = parseAcceptLanguage(acceptLang) ?? DEFAULT_LOCALE
+			}
+		}
+
+		// Load locales (hits cache after first request)
+		await loadLocale(DEFAULT_LOCALE)
+		if (detectedLocale !== DEFAULT_LOCALE) await loadLocale(detectedLocale)
+		locale.value = detectedLocale
+
+		const context: I18nContext = { locale, t, setLocale }
+		nuxtApp.vueApp.provide(I18N_INJECTION_KEY, context)
+	},
+})