feat(i18n): fallback via middleware (#8791)

packages/astro/src/@types/astro.ts

@@ -1379,6 +1379,21 @@ export interface AstroUserConfig {
 			 */
 			fallback?: Record<string, string[]>;
 
+			/**
+			 * @docs
+			 * @name experimental.i18n.fallbackControl
+			 * @type {"none" | "render" | "redirect"}
+			 * @version 3.*.*
+			 * @description
+			 *
+			 * Controls the fallback system of the internationalisation:
+			 *  - `none`: Astro will do nothing and will return `404` if a translated page isn't translated;
+			 *  - `redirect`: Astro will do a redirect to the fallback language if the translated page returns a `404`;
+			 *  - `render`: currently unsupported by Astro
+			 *
+			 */
+			fallbackControl: 'none' | 'render' | 'redirect';
+
 			/**
 			 * @docs
 			 * @name experimental.i18n.detectBrowserLanguage
@@ -1857,7 +1872,7 @@ export type AstroFeatureMap = {
 	/**
 	 * List of features that orbit around the i18n routing
 	 */
-	i18n?: AstroInternalisationFeature;
+	i18n?: AstroInternationalisationFeature;
 };
 
 export interface AstroAssetsFeature {
@@ -1872,7 +1887,7 @@ export interface AstroAssetsFeature {
 	isSquooshCompatible?: boolean;
 }
 
-export interface AstroInternalisationFeature {
+export interface AstroInternationalisationFeature {
 	/**
 	 * Wether the adapter is able to detect the language of the browser, usually using the `Accept-Language` header.
 	 */
@@ -2178,7 +2193,13 @@ export interface AstroPluginOptions {
 	logger: Logger;
 }
 
-export type RouteType = 'page' | 'endpoint' | 'redirect';
+/**
+ * - page: a route that lives in the file system, usually an Astro component
+ * - endpoint: a route that lives in the file system, usually a JS file that exposes endpoints methods
+ * - redirect: a route points to another route that lives in the file system
+ * - fallback: a route that doesn't exist in the file system that needs to be handled with other means, usually the middleware
+ */
+export type RouteType = 'page' | 'endpoint' | 'redirect' | 'fallback';
 
 export interface RoutePart {
 	content: string;
@@ -2207,7 +2228,6 @@ export interface RouteData {
 	prerender: boolean;
 	redirect?: RedirectConfig;
 	redirectRoute?: RouteData;
-	locale: string | undefined;
 }
 
 export type RedirectRouteData = RouteData & {

packages/astro/src/core/build/common.ts

@@ -25,6 +25,7 @@ export function getOutFolder(
 	switch (routeType) {
 		case 'endpoint':
 			return new URL('.' + appendForwardSlash(npath.dirname(pathname)), outRoot);
+		case 'fallback':
 		case 'page':
 		case 'redirect':
 			switch (astroConfig.build.format) {
@@ -52,6 +53,7 @@ export function getOutFile(
 		case 'endpoint':
 			return new URL(npath.basename(pathname), outFolder);
 		case 'page':
+		case 'fallback':
 		case 'redirect':
 			switch (astroConfig.build.format) {
 				case 'directory': {

packages/astro/src/core/config/schema.ts

@@ -278,29 +278,34 @@ export const AstroConfigSchema = z.object({
 						locales: z.string().array(),
 						fallback: z.record(z.string(), z.string().array()).optional().default({}),
 						detectBrowserLanguage: z.boolean().optional().default(false),
+						// TODO: properly add default when the feature goes of experimental
+						fallbackControl: z.enum(['none', 'redirect', 'render']).optional().default('none'),
 					})
 					.optional()
-					.refine((i18n) => {
+					.superRefine((i18n, ctx) => {
 						if (i18n) {
 							const { defaultLocale, locales, fallback } = i18n;
-							if (locales.includes(defaultLocale)) {
-								return {
+							if (!locales.includes(defaultLocale)) {
+								ctx.addIssue({
+									code: z.ZodIssueCode.custom,
 									message: `The default locale \`${defaultLocale}\` is not present in the \`i18n.locales\` array.`,
-								};
+								});
 							}
 							if (fallback) {
 								for (const [fallbackKey, fallbackArray] of Object.entries(fallback)) {
 									if (!locales.includes(fallbackKey)) {
-										return {
+										ctx.addIssue({
+											code: z.ZodIssueCode.custom,
 											message: `The locale \`${fallbackKey}\` key in the \`i18n.fallback\` record doesn't exist in the \`i18n.locales\` array.`,
-										};
+										});
 									}
 
 									for (const fallbackArrayKey of fallbackArray) {
 										if (!locales.includes(fallbackArrayKey)) {
-											return {
+											ctx.addIssue({
+												code: z.ZodIssueCode.custom,
 												message: `The locale \`${fallbackArrayKey}\` value in the \`i18n.fallback\` record doesn't exist in the \`i18n.locales\` array.`,
-											};
+											});
 										}
 									}
 								}

packages/astro/src/core/errors/errors-data.ts

@@ -1253,5 +1253,11 @@ export const MissingLocale = {
 	},
 } satisfies ErrorData;
 
+export const CantRenderPage = {
+	name: 'CantRenderPage',
+	title: "Astro can't render the route.",
+	message: "You tried to render a route that isn't a redirect and doesn't have any component.",
+} satisfies ErrorData;
+
 // Generic catch-all - Only use this in extreme cases, like if there was a cosmic ray bit flip
 export const UnknownError = { name: 'UnknownError', title: 'Unknown Error.' } satisfies ErrorData;

packages/astro/src/core/middleware/index.ts

@@ -1,8 +1,8 @@
-import type { MiddlewareResponseHandler, Params } from '../../@types/astro.js';
+import type { MiddlewareEndpointHandler, Params } from '../../@types/astro.js';
 import { createAPIContext } from '../endpoint/index.js';
 import { sequence } from './sequence.js';
 
-function defineMiddleware(fn: MiddlewareResponseHandler) {
+function defineMiddleware(fn: MiddlewareEndpointHandler) {
 	return fn;
 }
 

packages/astro/src/core/middleware/sequence.ts

@@ -1,4 +1,4 @@
-import type { APIContext, MiddlewareResponseHandler } from '../../@types/astro.js';
+import type { APIContext, MiddlewareEndpointHandler } from '../../@types/astro.js';
 import { defineMiddleware } from './index.js';
 
 // From SvelteKit: https://github.com/sveltejs/kit/blob/master/packages/kit/src/exports/hooks/sequence.js
@@ -6,10 +6,10 @@ import { defineMiddleware } from './index.js';
  *
  * It accepts one or more middleware handlers and makes sure that they are run in sequence.
  */
-export function sequence(...handlers: MiddlewareResponseHandler[]): MiddlewareResponseHandler {
+export function sequence(...handlers: MiddlewareEndpointHandler[]): MiddlewareEndpointHandler {
 	const length = handlers.length;
 	if (!length) {
-		const handler: MiddlewareResponseHandler = defineMiddleware((context, next) => {
+		const handler: MiddlewareEndpointHandler = defineMiddleware((context, next) => {
 			return next();
 		});
 		return handler;

packages/astro/src/core/pipeline.ts

@@ -67,7 +67,7 @@ export class Pipeline {
 	 */
 	async renderRoute(
 		renderContext: RenderContext,
-		componentInstance: ComponentInstance
+		componentInstance: ComponentInstance | undefined
 	): Promise<Response> {
 		const result = await this.#tryRenderRoute(
 			renderContext,
@@ -100,7 +100,8 @@ export class Pipeline {
 	async #tryRenderRoute<MiddlewareReturnType = Response>(
 		renderContext: Readonly<RenderContext>,
 		env: Readonly<Environment>,
-		mod: Readonly<ComponentInstance>,
+		mod: Readonly<ComponentInstance> | undefined,
+
 		onRequest?: MiddlewareHandler<MiddlewareReturnType>
 	): Promise<Response> {
 		const apiContext = createAPIContext({
@@ -113,6 +114,7 @@ export class Pipeline {
 
 		switch (renderContext.route.type) {
 			case 'page':
+			case 'fallback':
 			case 'redirect': {
 				if (onRequest) {
 					return await callMiddleware<Response>(
@@ -138,13 +140,7 @@ export class Pipeline {
 				}
 			}
 			case 'endpoint': {
-				const result = await callEndpoint(
-					mod as any as EndpointHandler,
-					env,
-					renderContext,
-					onRequest
-				);
-				return result;
+				return await callEndpoint(mod as any as EndpointHandler, env, renderContext, onRequest);
 			}
 			default:
 				throw new Error(`Couldn't find route of type [${renderContext.route.type}]`);

packages/astro/src/core/redirects/helpers.ts

@@ -9,6 +9,10 @@ export function routeIsRedirect(route: RouteData | undefined): route is Redirect
 	return route?.type === 'redirect';
 }
 
+export function routeIsFallback(route: RouteData | undefined): route is RedirectRouteData {
+	return route?.type === 'fallback';
+}
+
 export function redirectRouteGenerate(redirectRoute: RouteData, data: Params): string {
 	const routeData = redirectRoute.redirectRoute;
 	const route = redirectRoute.redirect;

packages/astro/src/core/render/context.ts

@@ -34,7 +34,7 @@ export type CreateRenderContextArgs = Partial<
 > & {
 	route: RouteData;
 	request: RenderContext['request'];
-	mod: ComponentInstance;
+	mod: ComponentInstance | undefined;
 	env: Environment;
 };
 

packages/astro/src/core/render/core.ts

@@ -13,9 +13,12 @@ import { redirectRouteGenerate, redirectRouteStatus, routeIsRedirect } from '../
 import type { RenderContext } from './context.js';
 import type { Environment } from './environment.js';
 import { createResult } from './result.js';
+import { AstroError } from '../errors/index.js';
+import { CantRenderPage } from '../errors/errors-data.js';
+import { routeIsFallback } from '../redirects/helpers.js';
 
 export type RenderPage = {
-	mod: ComponentInstance;
+	mod: ComponentInstance | undefined;
 	renderContext: RenderContext;
 	env: Environment;
 	cookies: AstroCookies;
@@ -29,6 +32,11 @@ export async function renderPage({ mod, renderContext, env, cookies }: RenderPag
 				location: redirectRouteGenerate(renderContext.route, renderContext.params),
 			},
 		});
+		// TODO: check this one
+	} else if (routeIsFallback(renderContext.route)) {
+		return new Response(null);
+	} else if (!mod) {
+		throw new AstroError(CantRenderPage);
 	}
 
 	// Validate the page component before rendering the page

packages/astro/src/core/render/params-and-props.ts

@@ -4,9 +4,10 @@ import type { Logger } from '../logger/core.js';
 import { routeIsRedirect } from '../redirects/index.js';
 import { getParams } from '../routing/params.js';
 import { RouteCache, callGetStaticPaths, findPathItemByKey } from './route-cache.js';
+import { routeIsFallback } from '../redirects/helpers.js';
 
 interface GetParamsAndPropsOptions {
-	mod: ComponentInstance;
+	mod: ComponentInstance | undefined;
 	route?: RouteData | undefined;
 	routeCache: RouteCache;
 	pathname: string;
@@ -26,11 +27,13 @@ export async function getParamsAndProps(opts: GetParamsAndPropsOptions): Promise
 	// This is a dynamic route, start getting the params
 	const params = getRouteParams(route, pathname) ?? {};
 
-	if (routeIsRedirect(route)) {
+	if (routeIsRedirect(route) || routeIsFallback(route)) {
 		return [params, {}];
 	}
 
-	validatePrerenderEndpointCollision(route, mod, params);
+	if (mod) {
+		validatePrerenderEndpointCollision(route, mod, params);
+	}
 
 	// During build, the route cache should already be populated.
 	// During development, the route cache is filled on-demand and may be empty.

packages/astro/src/core/render/route-cache.ts

@@ -16,7 +16,7 @@ import { validateDynamicRouteModule, validateGetStaticPathsResult } from '../rou
 import { generatePaginateFunction } from './paginate.js';
 
 interface CallGetStaticPathsOptions {
-	mod: ComponentInstance;
+	mod: ComponentInstance | undefined;
 	route: RouteData;
 	routeCache: RouteCache;
 	logger: Logger;
@@ -33,7 +33,9 @@ export async function callGetStaticPaths({
 	const cached = routeCache.get(route);
 	if (cached?.staticPaths) return cached.staticPaths;
 
-	validateDynamicRouteModule(mod, { ssr, route });
+	if (mod) {
+		validateDynamicRouteModule(mod, { ssr, route });
+	}
 
 	// No static paths in SSR mode. Return an empty RouteCacheEntry.
 	if (ssr && !route.prerender) {
@@ -42,22 +44,26 @@ export async function callGetStaticPaths({
 		return entry;
 	}
 
+	let staticPaths: GetStaticPathsResult = [];
 	// Add a check here to make TypeScript happy.
 	// This is already checked in validateDynamicRouteModule().
-	if (!mod.getStaticPaths) {
-		throw new Error('Unexpected Error.');
-	}
+	if (mod) {
+		if (!mod.getStaticPaths) {
+			throw new Error('Unexpected Error.');
+		}
 
-	// Calculate your static paths.
-	let staticPaths: GetStaticPathsResult = [];
-	staticPaths = await mod.getStaticPaths({
-		// Q: Why the cast?
-		// A: So users downstream can have nicer typings, we have to make some sacrifice in our internal typings, which necessitate a cast here
-		paginate: generatePaginateFunction(route) as PaginateFunction,
-		rss() {
-			throw new AstroError(AstroErrorData.GetStaticPathsRemovedRSSHelper);
-		},
-	});
+		if (mod) {
+			// Calculate your static paths.
+			staticPaths = await mod.getStaticPaths({
+				// Q: Why the cast?
+				// A: So users downstream can have nicer typings, we have to make some sacrifice in our internal typings, which necessitate a cast here
+				paginate: generatePaginateFunction(route) as PaginateFunction,
+				rss() {
+					throw new AstroError(AstroErrorData.GetStaticPathsRemovedRSSHelper);
+				},
+			});
+		}
+	}
 
 	validateGetStaticPathsResult(staticPaths, logger, route);
 

packages/astro/src/core/routing/index.ts

@@ -1,5 +1,5 @@
 export { createRouteManifest } from './manifest/create.js';
 export { deserializeRouteData, serializeRouteData } from './manifest/serialization.js';
-export { matchAllRoutes, matchRoute, matchDefaultLocaleRoutes } from './match.js';
+export { matchAllRoutes, matchRoute } from './match.js';
 export { getParams } from './params.js';
 export { validateDynamicRouteModule, validateGetStaticPathsResult } from './validation.js';

packages/astro/src/core/routing/manifest/create.ts

@@ -335,11 +335,6 @@ export function createRouteManifest(
 				const route = `/${segments
 					.map(([{ dynamic, content }]) => (dynamic ? `[${content}]` : content))
 					.join('/')}`.toLowerCase();
-				const locale = settings.config.experimental.i18n?.locales.find((currentLocale) => {
-					if (route.includes(`/${currentLocale}`)) {
-						return currentLocale;
-					}
-				});
 				routes.push({
 					route,
 					type: item.isPage ? 'page' : 'endpoint',
@@ -350,7 +345,6 @@ export function createRouteManifest(
 					generate,
 					pathname: pathname || undefined,
 					prerender,
-					locale,
 				});
 			}
 		});
@@ -413,11 +407,6 @@ export function createRouteManifest(
 					`An integration attempted to inject a route that is already used in your project: "${route}" at "${component}". \nThis route collides with: "${collision.component}".`
 				);
 			}
-			const locale = settings.config.experimental.i18n?.locales.find((currentLocale) => {
-				if (route.includes(`/${currentLocale}`)) {
-					return currentLocale;
-				}
-			});
 
 			// the routes array was already sorted by priority,
 			// pushing to the front of the list ensure that injected routes
@@ -432,7 +421,6 @@ export function createRouteManifest(
 				generate,
 				pathname: pathname || void 0,
 				prerender: prerenderInjected ?? prerender,
-				locale,
 			});
 		});
 
@@ -460,11 +448,6 @@ export function createRouteManifest(
 			.map(([{ dynamic, content }]) => (dynamic ? `[${content}]` : content))
 			.join('/')}`.toLowerCase();
 
-		const locale = settings.config.experimental.i18n?.locales.find((currentLocale) => {
-			if (route.includes(`/${currentLocale}`)) {
-				return currentLocale;
-			}
-		});
 		const routeData: RouteData = {
 			type: 'redirect',
 			route,
@@ -477,7 +460,6 @@ export function createRouteManifest(
 			prerender: false,
 			redirect: to,
 			redirectRoute: routes.find((r) => r.route === to),
-			locale,
 		};
 
 		const lastSegmentIsDynamic = (r: RouteData) => !!r.segments.at(-1)?.at(-1)?.dynamic;

packages/astro/src/core/routing/manifest/serialization.ts

@@ -32,6 +32,5 @@ export function deserializeRouteData(rawRouteData: SerializedRouteData): RouteDa
 		redirectRoute: rawRouteData.redirectRoute
 			? deserializeRouteData(rawRouteData.redirectRoute)
 			: undefined,
-		locale: undefined,
 	};
 }

packages/astro/src/core/routing/match.ts

@@ -1,4 +1,4 @@
-import type { AstroConfig, ManifestData, RouteData } from '../../@types/astro.js';
+import type { ManifestData, RouteData } from '../../@types/astro.js';
 
 /** Find matching route from pathname */
 export function matchRoute(pathname: string, manifest: ManifestData): RouteData | undefined {
@@ -9,59 +9,3 @@ export function matchRoute(pathname: string, manifest: ManifestData): RouteData
 export function matchAllRoutes(pathname: string, manifest: ManifestData): RouteData[] {
 	return manifest.routes.filter((route) => route.pattern.test(pathname));
 }
-
-/**
- * Given a pathname, the function attempts to retrieve the one that belongs to the `defaultLocale`.
- *
- * For example, given this configuration:
- *
- * ```js
- * {
- * 	defaultLocale: 'en',
- * 	locales: ['en', 'fr']
- * }
- * ```
- *
- * If we don't have the page `/fr/hello`, this function will attempt to match against `/en/hello`.
- */
-export function matchDefaultLocaleRoutes(
-	pathname: string,
-	manifest: ManifestData,
-	config: AstroConfig
-): RouteData[] {
-	// SAFETY: the function is called upon checking if `experimental.i18n` exists first
-	const i18n = config.experimental.i18n!;
-	const base = config.base;
-
-	const matchedRoutes: RouteData[] = [];
-	const defaultLocale = i18n.defaultLocale;
-
-	for (const route of manifest.routes) {
-		// we don't need to check routes that don't belong to the default locale
-		if (route.locale === defaultLocale) {
-			// we check if the current route pathname contains `/en` somewhere
-			if (
-				route.pathname?.startsWith(`/${defaultLocale}`) ||
-				route.pathname?.startsWith(`${base}/${defaultLocale}`)
-			) {
-				let localeToReplace;
-				// now we need to check if the locale inside `pathname` is actually one of the locales configured
-				for (const locale of i18n.locales) {
-					if (pathname.startsWith(`${base}/${locale}`) || pathname.startsWith(`/${locale}`)) {
-						localeToReplace = locale;
-						break;
-					}
-				}
-				if (localeToReplace) {
-					// we attempt the replace the locale found with the default locale, and now we could if matches the current `route`
-					const maybePathname = pathname.replace(localeToReplace, defaultLocale);
-					if (route.pattern.test(maybePathname)) {
-						matchedRoutes.push(route);
-					}
-				}
-			}
-		}
-	}
-
-	return matchedRoutes;
-}

packages/astro/src/i18n/middleware.ts

@@ -0,0 +1,38 @@
+import type { AstroConfig, MiddlewareEndpointHandler } from '../@types/astro.js';
+import type { Logger } from '../core/logger/core.js';
+
+export function createI18nMiddleware(
+	config: Readonly<AstroConfig>,
+	logger: Logger
+): MiddlewareEndpointHandler | undefined {
+	const i18n = config.experimental?.i18n;
+	if (!i18n) {
+		return undefined;
+	}
+	const fallbackKeys = Object.keys(i18n.fallback);
+	const locales = i18n.locales;
+
+	logger.debug('i18n', 'Successfully created middleware');
+	return async (context, next) => {
+		if (fallbackKeys.length <= 0) {
+			return next();
+		}
+
+		const response = await next();
+		if (i18n.fallbackControl === 'redirect' && response instanceof Response) {
+			const url = context.url;
+			const separators = url.pathname.split('/');
+
+			const urlLocale = separators.find((s) => locales.includes(s));
+
+			if (urlLocale && fallbackKeys.includes(urlLocale)) {
+				// TODO: correctly handle chain of fallback
+				const fallbackLocale = i18n.fallback[urlLocale][0];
+				const newPathname = url.pathname.replace(`/${urlLocale}`, `/${fallbackLocale}`);
+				return context.redirect(newPathname);
+			}
+		}
+
+		return response;
+	};
+}

packages/astro/src/vite-plugin-astro-server/route.ts

@@ -9,9 +9,14 @@ import type {
 } from '../@types/astro.js';
 import { AstroErrorData, isAstroError } from '../core/errors/index.js';
 import { loadMiddleware } from '../core/middleware/loadMiddleware.js';
-import { createRenderContext, getParamsAndProps, type SSROptions } from '../core/render/index.js';
+import {
+	createRenderContext,
+	getParamsAndProps,
+	type RenderContext,
+	type SSROptions,
+} from '../core/render/index.js';
 import { createRequest } from '../core/request.js';
-import { matchAllRoutes, matchDefaultLocaleRoutes } from '../core/routing/index.js';
+import { matchAllRoutes } from '../core/routing/index.js';
 import { isPage } from '../core/util.js';
 import { getSortedPreloadedMatches } from '../prerender/routing.js';
 import { isServerLikeOutput } from '../prerender/utils.js';
@@ -23,6 +28,8 @@ import { preload } from './index.js';
 import { getComponentMetadata } from './metadata.js';
 import { handle404Response, writeSSRResult, writeWebResponse } from './response.js';
 import { getScriptsForURL } from './scripts.js';
+import { createI18nMiddleware } from '../i18n/middleware.js';
+import { sequence } from '../core/middleware/index.js';
 
 const clientLocalsSymbol = Symbol.for('astro.locals');
 
@@ -51,14 +58,9 @@ export async function matchRoute(
 	pipeline: DevPipeline
 ): Promise<MatchedRoute | undefined> {
 	const env = pipeline.getEnvironment();
-	const config = pipeline.getConfig();
 	const { routeCache, logger } = env;
 	let matches = matchAllRoutes(pathname, manifestData);
 
-	// if we haven't found any match, we try to fetch the default locale matched route
-	if (matches.length === 0 && config.experimental.i18n) {
-		matches = matchDefaultLocaleRoutes(pathname, manifestData, config);
-	}
 	const preloadedMatches = await getSortedPreloadedMatches({
 		pipeline,
 		matches,
@@ -163,70 +165,133 @@ export async function handleRoute({
 	const config = pipeline.getConfig();
 	const moduleLoader = pipeline.getModuleLoader();
 	const { logger } = env;
-	if (!matchedRoute) {
+	if (!matchedRoute && !config.experimental.i18n) {
 		return handle404Response(origin, incomingRequest, incomingResponse);
 	}
 
-	const filePath: URL | undefined = matchedRoute.filePath;
-	const { route, preloadedComponent } = matchedRoute;
 	const buildingToSSR = isServerLikeOutput(config);
 
-	// Headers are only available when using SSR.
-	const request = createRequest({
-		url,
-		headers: buildingToSSR ? incomingRequest.headers : new Headers(),
-		method: incomingRequest.method,
-		body,
-		logger,
-		ssr: buildingToSSR,
-		clientAddress: buildingToSSR ? incomingRequest.socket.remoteAddress : undefined,
-		locals: Reflect.get(incomingRequest, clientLocalsSymbol), // Allows adapters to pass in locals in dev mode.
-	});
-
-	// Set user specified headers to response object.
-	for (const [name, value] of Object.entries(config.server.headers ?? {})) {
-		if (value) incomingResponse.setHeader(name, value);
-	}
-
-	const options: SSROptions = {
-		env,
-		filePath,
-		preload: preloadedComponent,
-		pathname,
-		request,
-		route,
-	};
+	let request: Request;
+	let renderContext: RenderContext;
+	let mod: ComponentInstance | undefined = undefined;
+	let options: SSROptions | undefined = undefined;
+	let route: RouteData;
 	const middleware = await loadMiddleware(moduleLoader, settings.config.srcDir);
-	if (middleware) {
-		options.middleware = middleware;
+
+	if (!matchedRoute) {
+		if (config.experimental.i18n) {
+			const locales = config.experimental.i18n.locales;
+			const pathNameHasLocale = pathname
+				.split('/')
+				.filter(Boolean)
+				.some((segment) => {
+					return locales.includes(segment);
+				});
+			if (!pathNameHasLocale) {
+				return handle404Response(origin, incomingRequest, incomingResponse);
+			}
+			request = createRequest({
+				url,
+				headers: buildingToSSR ? incomingRequest.headers : new Headers(),
+				logger,
+				ssr: buildingToSSR,
+			});
+			route = {
+				component: '',
+				generate(_data: any): string {
+					return '';
+				},
+				params: [],
+				pattern: new RegExp(''),
+				prerender: false,
+				segments: [],
+				type: 'fallback',
+				route: '',
+			};
+			renderContext = await createRenderContext({
+				request,
+				pathname,
+				env,
+				mod,
+				route,
+			});
+		} else {
+			return handle404Response(origin, incomingRequest, incomingResponse);
+		}
+	} else {
+		const filePath: URL | undefined = matchedRoute.filePath;
+		const { preloadedComponent } = matchedRoute;
+		route = matchedRoute.route;
+		// Headers are only available when using SSR.
+		request = createRequest({
+			url,
+			headers: buildingToSSR ? incomingRequest.headers : new Headers(),
+			method: incomingRequest.method,
+			body,
+			logger,
+			ssr: buildingToSSR,
+			clientAddress: buildingToSSR ? incomingRequest.socket.remoteAddress : undefined,
+			locals: Reflect.get(incomingRequest, clientLocalsSymbol), // Allows adapters to pass in locals in dev mode.
+		});
+
+		// Set user specified headers to response object.
+		for (const [name, value] of Object.entries(config.server.headers ?? {})) {
+			if (value) incomingResponse.setHeader(name, value);
+		}
+
+		options = {
+			env,
+			filePath,
+			preload: preloadedComponent,
+			pathname,
+			request,
+			route,
+		};
+		if (middleware) {
+			options.middleware = middleware;
+		}
+
+		mod = options.preload;
+
+		const { scripts, links, styles, metadata } = await getScriptsAndStyles({
+			pipeline,
+			filePath: options.filePath,
+		});
+
+		renderContext = await createRenderContext({
+			request: options.request,
+			pathname: options.pathname,
+			scripts,
+			links,
+			styles,
+			componentMetadata: metadata,
+			route: options.route,
+			mod,
+			env,
+		});
 	}
-	const mod = options.preload;
 
-	const { scripts, links, styles, metadata } = await getScriptsAndStyles({
-		pipeline,
-		filePath: options.filePath,
-	});
+	const onRequest = middleware?.onRequest as MiddlewareEndpointHandler | undefined;
+	if (config.experimental.i18n) {
+		const i18Middleware = createI18nMiddleware(config, logger);
 
-	const renderContext = await createRenderContext({
-		request: options.request,
-		pathname: options.pathname,
-		scripts,
-		links,
-		styles,
-		componentMetadata: metadata,
-		route: options.route,
-		mod,
-		env,
-	});
-	const onRequest = options.middleware?.onRequest as MiddlewareEndpointHandler | undefined;
-	if (onRequest) {
+		if (i18Middleware) {
+			if (onRequest) {
+				pipeline.setMiddlewareFunction(sequence(i18Middleware, onRequest));
+			} else {
+				pipeline.setMiddlewareFunction(i18Middleware);
+			}
+		} else if (onRequest) {
+			pipeline.setMiddlewareFunction(onRequest);
+		}
+	} else if (onRequest) {
 		pipeline.setMiddlewareFunction(onRequest);
 	}
 
 	let response = await pipeline.renderRoute(renderContext, mod);
 	if (response.status === 404 && has404Route(manifestData)) {
 		const fourOhFourRoute = await matchRoute('/404', manifestData, pipeline);
-		if (fourOhFourRoute?.route !== options.route)
+		if (options && fourOhFourRoute?.route !== options.route)
 			return handleRoute({
 				...options,
 				matchedRoute: fourOhFourRoute,

packages/astro/test/dev-routing.test.js

@@ -377,10 +377,9 @@ describe('Development Routing', () => {
 			expect(await response2.text()).includes('Hola mundo');
 		});
 
-		it("should render the default locale if there isn't a fallback and the route is missing", async () => {
+		it("should NOT render the default locale if there isn't a fallback and the route is missing", async () => {
 			const response = await fixture.fetch('/it/start');
-			expect(response.status).to.equal(200);
-			expect(await response.text()).includes('Hello');
+			expect(response.status).to.equal(404);
 		});
 
 		it("should render a 404 because the route `fr` isn't included in the list of locales of the configuration", async () => {
@@ -426,7 +425,65 @@ describe('Development Routing', () => {
 			expect(await response2.text()).includes('Hola mundo');
 		});
 
-		it("should render the default locale if there isn't a fallback and the route is missing", async () => {
+		it("should NOT render the default locale if there isn't a fallback and the route is missing", async () => {
+			const response = await fixture.fetch('/new-site/it/start');
+			expect(response.status).to.equal(404);
+		});
+
+		it("should render a 404 because the route `fr` isn't included in the list of locales of the configuration", async () => {
+			const response = await fixture.fetch('/new-site/fr/start');
+			expect(response.status).to.equal(404);
+		});
+	});
+
+	describe('i18n routing with fallback [redirect]', () => {
+		/** @type {import('./test-utils').Fixture} */
+		let fixture;
+		/** @type {import('./test-utils').DevServer} */
+		let devServer;
+
+		before(async () => {
+			fixture = await loadFixture({
+				root: './fixtures/i18n-routing-fallback/',
+				experimental: {
+					i18n: {
+						defaultLocale: 'en',
+						locales: ['en', 'pt', 'it'],
+						fallback: {
+							it: ['en'],
+						},
+						fallbackControl: 'redirect',
+					},
+				},
+			});
+			devServer = await fixture.startDevServer();
+		});
+
+		after(async () => {
+			await devServer.stop();
+		});
+
+		it('should render the en locale', async () => {
+			const response = await fixture.fetch('/new-site/en/start');
+			expect(response.status).to.equal(200);
+			expect(await response.text()).includes('Hello');
+
+			const response2 = await fixture.fetch('/new-site/en/blog/1');
+			expect(response2.status).to.equal(200);
+			expect(await response2.text()).includes('Hello world');
+		});
+
+		it('should render localised page correctly', async () => {
+			const response = await fixture.fetch('/new-site/pt/start');
+			expect(response.status).to.equal(200);
+			expect(await response.text()).includes('Hola');
+
+			const response2 = await fixture.fetch('/new-site/pt/blog/1');
+			expect(response2.status).to.equal(200);
+			expect(await response2.text()).includes('Hola mundo');
+		});
+
+		it('should render the english locale, which is the first fallback', async () => {
 			const response = await fixture.fetch('/new-site/it/start');
 			expect(response.status).to.equal(200);
 			expect(await response.text()).includes('Hello');

packages/astro/test/fixtures/i18n-routing-base/astro.config.mjs

@@ -7,10 +7,7 @@ export default defineConfig({
 			defaultLocale: 'en',
 			locales: [
 				'en', 'pt', 'it'
-			],
-			fallback: {
-				'pt-BR': ['pt']
-			}
+			]
 		}
 	}
 })

packages/astro/test/fixtures/i18n-routing-fallback/astro.config.mjs

@@ -0,0 +1,16 @@
+import { defineConfig} from "astro/config";
+
+export default defineConfig({
+    base: "new-site",
+    experimental: {
+        i18n: {
+            defaultLocale: 'en',
+            locales: [
+                'en', 'pt', 'it'
+            ],
+            fallback: {
+                "it": ["en"]
+            }
+        }
+    }
+})

packages/astro/test/fixtures/i18n-routing-fallback/package.json

@@ -0,0 +1,8 @@
+{
+  "name": "@test/i18n-routing-fallabck",
+  "version": "0.0.0",
+  "private": true,
+  "dependencies": {
+    "astro": "workspace:*"
+  }
+}

packages/astro/test/fixtures/i18n-routing-fallback/src/pages/en/blog/[id].astro

@@ -0,0 +1,18 @@
+---
+export function getStaticPaths() {
+    return [
+        {params: {id: '1'}, props: { content: "Hello world" }},
+        {params: {id: '2'}, props: { content: "Eat Something" }},
+        {params: {id: '3'}, props: { content: "How are you?" }},
+    ];
+}
+const { content } = Astro.props;
+---
+<html>
+<head>
+    <title>Astro</title>
+</head>
+<body>
+{content}
+</body>
+</html>

packages/astro/test/fixtures/i18n-routing-fallback/src/pages/en/start.astro

@@ -0,0 +1,8 @@
+<html>
+<head>
+	<title>Astro</title>
+</head>
+<body>
+Hello
+</body>
+</html>

packages/astro/test/fixtures/i18n-routing-fallback/src/pages/index.astro

@@ -0,0 +1,8 @@
+<html>
+<head>
+	<title>Astro</title>
+</head>
+<body>
+	Hello
+</body>
+</html>

packages/astro/test/fixtures/i18n-routing-fallback/src/pages/pt/blog/[id].astro

@@ -0,0 +1,18 @@
+---
+export function getStaticPaths() {
+    return [
+        {params: {id: '1'}, props: { content: "Hola mundo" }},
+        {params: {id: '2'}, props: { content: "Eat Something" }},
+        {params: {id: '3'}, props: { content: "How are you?" }},
+    ];
+}
+const { content } = Astro.props;
+---
+<html>
+<head>
+    <title>Astro</title>
+</head>
+<body>
+{content}
+</body>
+</html>

packages/astro/test/fixtures/i18n-routing-fallback/src/pages/pt/start.astro

@@ -0,0 +1,8 @@
+<html>
+<head>
+	<title>Astro</title>
+</head>
+<body>
+Hola
+</body>
+</html>

packages/astro/test/fixtures/i18n-routing/astro.config.mjs

@@ -6,10 +6,7 @@ export default defineConfig({
 			defaultLocale: 'en',
 			locales: [
 				'en', 'pt', 'it'
-			],
-			fallback: {
-				'pt-BR': ['pt']
-			}
+			]
 		}
 	},
 	base: "/new-site"

packages/astro/test/units/config/config-validate.test.js

@@ -77,4 +77,66 @@ describe('Config Validation', () => {
 			'The value of `outDir` must not point to a path within the folder set as `publicDir`, this will cause an infinite loop'
 		);
 	});
+
+	describe('i18n', async () => {
+		it('defaultLocale is not in locales', async () => {
+			const configError = await validateConfig(
+				{
+					experimental: {
+						i18n: {
+							defaultLocale: 'en',
+							locales: ['es'],
+						},
+					},
+				},
+				process.cwd()
+			).catch((err) => err);
+			expect(configError instanceof z.ZodError).to.equal(true);
+			expect(configError.errors[0].message).to.equal(
+				'The default locale `en` is not present in the `i18n.locales` array.'
+			);
+		});
+
+		it('errors if a fallback value does not exist', async () => {
+			const configError = await validateConfig(
+				{
+					experimental: {
+						i18n: {
+							defaultLocale: 'en',
+							locales: ['es', 'en'],
+							fallback: {
+								es: ['it'],
+							},
+						},
+					},
+				},
+				process.cwd()
+			).catch((err) => err);
+			expect(configError instanceof z.ZodError).to.equal(true);
+			expect(configError.errors[0].message).to.equal(
+				"The locale `it` value in the `i18n.fallback` record doesn't exist in the `i18n.locales` array."
+			);
+		});
+
+		it('errors if a fallback key does not exist', async () => {
+			const configError = await validateConfig(
+				{
+					experimental: {
+						i18n: {
+							defaultLocale: 'en',
+							locales: ['es', 'en'],
+							fallback: {
+								it: ['en'],
+							},
+						},
+					},
+				},
+				process.cwd()
+			).catch((err) => err);
+			expect(configError instanceof z.ZodError).to.equal(true);
+			expect(configError.errors[0].message).to.equal(
+				"The locale `it` key in the `i18n.fallback` record doesn't exist in the `i18n.locales` array."
+			);
+		});
+	});
 });

pnpm-lock.yaml

@@ -2767,6 +2767,12 @@ importers:
         specifier: workspace:*
         version: link:../../..
 
+  packages/astro/test/fixtures/i18n-routing-fallback:
+    dependencies:
+      astro:
+        specifier: workspace:*
+        version: link:../../..
+
   packages/astro/test/fixtures/import-ts-with-js:
     dependencies:
       astro: