fix(i18n): emit an error when the index isn't found (#9678)

* fix(i18n): emit an error when the index isn't found

* changeset

* Update .changeset/proud-guests-bake.md

Co-authored-by: Sarah Rainsberger <sarah@rainsberger.ca>

* rename

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

Co-authored-by: Florian Lefebvre <contact@florian-lefebvre.dev>

---------

Co-authored-by: Sarah Rainsberger <sarah@rainsberger.ca>
Co-authored-by: Florian Lefebvre <contact@florian-lefebvre.dev>

.changeset/proud-guests-bake.md

@@ -0,0 +1,5 @@
+---
+"astro": patch
+---
+
+Adds an error during the build phase in case `i18n.routing.prefixDefaultLocale` is set to `true` and the index page is missing.

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

@@ -1001,6 +1001,13 @@ export const MissingLocale = {
 		`The locale/path \`${locale}\` does not exist in the configured \`i18n.locales\`.`,
 } satisfies ErrorData;
 
+export const MissingIndexForInternationalization = {
+	name: 'MissingIndexForInternationalizationError',
+	title: 'Index page not found.',
+	message: (src: string) =>
+		`Astro couldn't find the index URL. This index page is required to create a redirect from the index URL to the index URL of the default locale. \nCreate an index page in \`${src}\``,
+} satisfies ErrorData;
+
 /**
  * @docs
  * @description

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

@@ -18,6 +18,8 @@ import { SUPPORTED_MARKDOWN_FILE_EXTENSIONS } from '../../constants.js';
 import { removeLeadingForwardSlash, slash } from '../../path.js';
 import { resolvePages } from '../../util.js';
 import { getRouteGenerator } from './generator.js';
+import { AstroError } from '../../errors/index.js';
+import { MissingIndexForInternationalization } from '../../errors/errors-data.js';
 const require = createRequire(import.meta.url);
 
 interface Item {
@@ -513,6 +515,21 @@ export function createRouteManifest(
 	});
 	const i18n = settings.config.i18n;
 	if (i18n) {
+		// First we check if the user doesn't have an index page.
+		if (i18n.routing === 'prefix-always') {
+			let index = routes.find((route) => route.route === '/');
+			if (!index) {
+				let relativePath = path.relative(
+					fileURLToPath(settings.config.root),
+					fileURLToPath(new URL('pages', settings.config.srcDir))
+				);
+				throw new AstroError({
+					...MissingIndexForInternationalization,
+					message: MissingIndexForInternationalization.message(relativePath),
+				});
+			}
+		}
+
 		// In this block of code we group routes based on their locale
 
 		// A map like: locale => RouteData[]